RU2763288C1 - Heat-accumulating composition based on eutectic mixture of crystal hydrates of calcium and cadmium nitrates - Google Patents

Abstract

FIELD: heat-saving technologies; energy-saving technologies.

SUBSTANCE: heat-accumulating composition, based on an eutectic mixtures of calcium nitrate tetrahydrate and cadmium nitrate tetrahydrate, contains an eutectic mixture of Ca(NO₃)₂·4H₂O and Cd(NO₃)₂·4H₂O with the addition of expanded graphite and carboxymethylcellulose, or an eutectic mixture of Ca(NO₃)₂·4H₂O and Cd(NO₃)₂·4H₂O with the addition of calcium oxide and carboxymethylcellulose, or an eutectic mixture of Ca(NO₃)₂·4H₂O and Cd(NO₃)₂·4H₂O with the addition of calcium hydroxide and carboxymethylcellulose. The heat-accumulating composition is characterized by a temperature of a phase transition, which ensures the operability of the composition as heat-accumulating material, between 36 and 45°C. The specified composition is prepared by heating to 60°C of the eutectic mixture of Ca(NO₃)₂·4H₂O and Cd(NO₃)₂·4H₂O, melting for 30 min with constant mixing, holding for 10 min, sequential addition of additives and further mixing for 3 h with a temperature control of 60°C.

EFFECT: invention makes it possible to obtain material for accumulating excess heat energy for the purpose of heating rooms, motor vehicle salons.

1 cl, 9 dwg, 1 tbl, 3 ex

Description

The invention relates to heat storage materials that store and release heat in the temperature range of 36-45°C due to phase transition processes and can be used to accumulate excess thermal energy to heat rooms, vehicle interiors and maintain temperature. It is possible to introduce the proposed heat storage material into the floor heating system to ensure optimal temperature conditions during the cold periods of the year.

Known "Heat storage material" (patent SU 867918 A1, priority date: 10/25/1979) based on crystalline hydrates of cadmium and nickel nitrates in the ratio 45-55:55-45 with a eutectic melting point of 37.5°C, which is 2-6°C lower than the proposed compositions based on crystalline hydrates of calcium and cadmium nitrates and the heat of fusion 140-145 J/g, which is approximately 20-24 J/g higher than the proposed compositions based on crystalline hydrates of calcium and cadmium nitrates (depending on the ratio of components ). The authors did not show the levels of supercooling of this material and there is no information on the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density of the liquid phase, heat accumulation density, total enthalpy in the operating temperature range and accumulation time, which determines the performance of the material, without which it is impossible to design a heat accumulator based on TAM. Therefore, it is impossible to evaluate the efficiency of a heat storage material based on crystalline hydrates of cadmium and nickel nitrates, and it is impossible to calculate the parameters of heat accumulators based on it.

Another composition proposed by the authors in the patent for invention SU 834088 A1 (priority date: 05/30/1979) contains crystalline hydrates of calcium and cobalt nitrates in the ratio 45-55:55-45. The melting point of this composition is 29.1-29.3°C with a heat of fusion of 128-135 J/g. The phase transition temperature of this material does not correspond to the declared temperature range. Also, hydrated cobalt nitrate begins to decompose at 50°C, so it is difficult to control a constant composition at a high content of hydrated cobalt nitrate. Also, the authors do not show the levels of supercooling of this material and there is no information on the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range and accumulation time, which determines the material's performance.

Known "Heat storage composition based on hydrated zinc nitrate" (SU 983134 A1, priority date: 21.05.1981) containing 0.5-5% hydrated cobalt nitrate and 99.5-95% hydrated zinc nitrate. This composition is not eutectic. The authors used cobalt nitrate crystalline hydrate as an additive to reduce hypothermia. The authors managed to reduce the supercooling to 5-7°C at a melting point of 36.4°C. However, data on the enthalpy of fusion are not available. Another significant drawback is the lack of information about the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range and accumulation time, which determines the performance of the material.

A composition of a more complex structure based on nitrate crystalline hydrates is proposed in the patent for invention RU 2567921 C1 "Heat storage material" (priority date: 04/29/2014). The authors used to prepare a mixture of crystalline nitrates of zinc, nickel, magnesium and lithium in ratios of 4.5-6.5:10.5-14.5:16.5-18.5:68.5-60.5 mass percent, respectively. The enthalpy of melting was 220 J/g at a melting temperature of 25.5°С. The phase transition temperature of this material does not correspond to the declared temperature range. Subcooling does not exceed 4°C. The disadvantages of this patent include the lack of information about the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range and accumulation time, which determines the performance of the material.

A known method for obtaining a heat storage material is described in patent EP 1156097 B1 (priority date 10/15/2003), which proposes an improved method for vacuum impregnation of a graphite matrix with a phase transition material. The authors indicate the possibility of using Ca(NO ₃ ) ₂ ⋅4H ₂ O or Cd(NO ₃ ) ₂ ⋅4H ₂ O. Such important characteristics as the levels of supercooling of the composition, the presence / absence of phase segregation, dynamic and kinematic phase, density, heat storage density, total enthalpy in the operating temperature range and accumulation time, which determines the performance of the material, are not indicated, so it is difficult to determine the performance of the proposed composition.

Known patent US 9914865 B2 (priority date 03/13/2018), in which Ca(NO ₃ ) ₂ ⋅4H ₂ O and Cd(NO ₃ ) ₂ ⋅4H ₂ O were also considered as starting materials for obtaining heat storage compositions. The quantitative content of Ca(NO ₃ ) ₂ ⋅4H ₂ O and Cd(NO ₃ ) ₂ ⋅4H ₂ O in compositions containing these crystalline hydrates together with complex organics, ionic liquids, is not indicated. Also, the levels of supercooling of this material, the presence / absence of phase segregation, density, heat storage density, total enthalpy in the operating temperature range and accumulation time, which determines the performance of the material, are not shown. Because of this, it is difficult to determine the performance of the compositions.

Known patent WO 2020074883 A1 (priority date 04/16/2000), in which the authors consider the possibility of creating heat-storage compositions by a combination of salts, including crystalline hydrates of group 2 metals, including Ca(NO ₃ ) ₂ ⋅4H ₂ O and LiNO ₃ ⋅3H ₂ O as the main components of the heat storage composition. The authors indicate that the content of Ca(NO ₃ ) ₂ ⋅4H ₂ O and LiNO ₃ ⋅3H ₂ O varies from 40% to 95%. Crystallization of this composition, after adding the nucleating agent was at the level of 18°C, which does not correspond to the stated temperature range of 36-45°C. Also, the values of the enthalpy of melting, phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range, and accumulation time, which determines the performance of the material, are not considered.

Patent DE 59702643 D1 (priority date 12/21/2000) proposes a phase transition material for heat and cold storage from -25 to 150°C, containing a graphite matrix and a phase transition material. The disadvantage of this invention is that the levels of supercooling of this material are not shown, and there is no information about the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range and time accumulation, which determines the performance of the material.

Closest to the proposed invention in terms of essential features is the composition described in the patent SU 943265 A1 (priority date: 20.10.1980). It proposes to use crystalline hydrates of cadmium and zinc nitrates in a ratio of 35-45:65-55. This composition has a phase transition temperature of 27.4°C and a heat of fusion of 123–129 J/g. The phase transition temperature of this material does not correspond to the declared temperature range. The disadvantage of this invention is that the levels of supercooling of this material are not shown, and there is no information about the presence / absence of phase segregation, dynamic and kinematic viscosities, heat capacity of the liquid phase, density, heat storage density, total enthalpy in the operating temperature range and time accumulation, which determines the performance of the material.

Thus, the disadvantages of all the above patents include the lack of experimental data on density, viscosity, heat capacity, thermal conductivity and energy storage density, and the difficulty of their calculations due to the lack of information about the method of studying materials and graphic data. In addition, only in some works, hypothermia is considered as a factor influencing the efficiency of the TAM operation. However, none of the works gives the accumulative capacity of the obtained mixtures.

The objective of this invention is to eliminate incongruent melting, phase segregation and minimization of supercooling, which would not exceed 3°C, of heat storage materials based on a mixture of calcium and cadmium nitrate crystalline hydrates.

This problem is solved by preparing a eutectic mixture of crystalline hydrates, the phase transition temperature of which is reduced compared to the temperatures of pure components, and thus incogruent melting is replaced by congruent one. The introduction of special additives makes it possible to get rid of phase segregation and minimize overcooling so that the heat storage material in the melting/crystallization cycle falls within the required temperature range.

When implementing this invention, a technical result is created, which consists in the absence of phase segregation, congruent melting in a phase-transition heat-storage material, an increase in thermal stability, at a phase transition temperature from 36 to 45 ° C., a decrease in supercooling, a high phase transition enthalpy, environmental safety. Various characteristics were measured and calculated for the compositions, which characterize the efficiency and stability of the heat storage composition at the stages of melting and crystallization.

The technical result is achieved due to the fact that the heat storage composition includes additional substances - calcium oxide, expanded graphite, calcium hydroxide and carboxymethyl cellulose as nucleating agents and a thickener. The main problems of using crystalline hydrates in their pure form are incongruent melting and supercooling. The addition of a second crystalline hydrate to create a eutectic mixture only partially eliminates this problem, so it is proposed to use additional substances, namely thickeners to increase the viscosity and nucleating agents. The most commonly used thickeners are carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA), which prevent phase segregation of materials during slow melting. The nucleating agents are selected in such a way as to provide the melt with crystallization epicenters. For this, substances are usually selected that have a similar crystal lattice and do not melt in the operating temperature range of the heat storage material. Often, oxides of the corresponding metals and various modifications of carbon are used for these purposes, most often soot and heat-treated graphite. The use of all components in the required quantitative ratios allows the use of heat storage materials in a cyclic mode in the temperature range of the phase transition from 36 to 45°C, depending on the composition for heating purposes as a low-temperature layer.

As the proposed heat storage materials, it is proposed to use mixtures consisting of:

- Calcium nitrate tetrahydrate Ca(NO ₃ ) ₂ ⋅4H ₂ O with mass. share of 70%;

- Cadmium nitrate tetrahydrate Ca(NO ₃ ) ₂ ⋅4H ₂ O with mass. share of 30%;

- Expanded graphite with mass. shares in relation to the eutectic composition of 1% (1%C _eg );

- Calcium oxide CaO with mass. shares in relation to the eutectic composition of 1%;

- Calcium hydroxide Ca(OH) ₂ with mass. shares in relation to the eutectic composition of 1%;

- Carboxymethylcellulose with mass. volume fraction in relation to the eutectic composition 1% (1% CMC).

According to the NFPA 704 classifier, cadmium nitrate tetrahydrate is non-flammable and stable under increasing pressure and temperature. The decomposition of nitrate crystal hydrate occurs until the complete elimination of water and the subsequent release of CdO, NO ₂ , O ₂ (Kozub P.A., Kozub S.V. Research and technology: Processing of small-sized nickel-cadmium batteries (Russian Edition). // Germany: LAP LAMBERT Academic Publishing. - 2015. - 152 pp.) at 350°C (Inorganic and organoelement compounds. // Ed. 7th, trans. and additional. In three volumes. Volume Sh. Edited by Prof. N.V. Lazarev and Doctor of Biological Sciences Prof. ID Gadaskina (L.: "Chemistry" - 1977. - 608 p.), but under operating conditions, heating to this temperature is impossible and does not exceed 80°C. However, direct contact with cadmium salts inside the body should be avoided, as this is very dangerous and can lead to death. Therefore, it is necessary to use the material in closed hermetic systems in order to avoid contact with a hazardous substance. Compliance with all safety requirements will eliminate the danger from cadmium nitrate, so the operation of materials based on it can be safe.

An experimental evaluation of the implementation of the proposed invention was carried out on the example of the analysis of three compositions of heat storage materials. For the compositions under consideration, the kinematic and dynamic viscosities, solid phase density, energy storage density, accumulation time, crystallization onset temperature were studied. Most of the parameters are measured and calculated based on the temperature history method, which simulates the thermodynamic behavior of compositions under real conditions.

The invention is illustrated by the following drawings:

fig. 1 - DSC curve for heat storage material composition 1;

fig. 2 - temperature history curve for the heat storage material of composition 1;

fig. 3 - viscosity of the heat storage material of composition 1;

fig. 4 - DSC curve for heat storage material composition 2;

fig. 5 is a temperature history curve for the thermal storage material of composition 2;

fig. 6 - viscosity of the heat storage material a of composition 2;

fig. 7 - DSC curve for the heat storage material of composition 3;

fig. 8 is a temperature history curve for the thermal storage material of composition 3;

fig. 9 - viscosity of the heat storage material of composition 3.

Composition 1;

(70% Ca (NO ₃ ) ₂ ⋅4H ₂ O + 30% Cd (NO ₃ ) ₂ ⋅ 4H ₂ O) + 1% EG + 1% CMC

Crystalline hydrates of calcium and cadmium nitrates weighing 14 and 6 g (2 identical mixtures were prepared), respectively, were weighed in a ratio of 70:30 and heated to 60°C, after which they were melted for 30 minutes with constant stirring until complete melting, and then kept in within 10 minutes. Then, for effective nucleation and prevention of phase segregation, 1% of expanded graphite (EG) and carboxymethyl cellulose (CMC) (0.2 g each) were added. All ratios, with the exception of CMC, are taken by weight. Carboxymethylcellulose as a thickener is taken in volume fraction. Within three hours after the addition, the mixture was stirred at a maximum heating temperature of 60°C.

To confirm the properties of the synthesized materials by differential scanning calorimetry (DSC), the temperature and heat of melting, as well as the magnitude of the temperature hysteresis, were studied. Experimental conditions by DSC method:

• Maximum heating temperature, °С, 70;

• Minimum cooling temperature, °С, 0;

• Heating rate, °С/min: 10;

• Atmosphere, N ₂ ;

• Cooling rate, °С/min: 2;

• Gas for cooling, N ₂ ;

• Gas supply rate ml/min, 40.

In FIG. Figure 1 shows the DSC curve for this composition with a melting heat of 124.9 J/g and a phase transition onset temperature of 43.42°C.

In FIG. 2 is a graph of the temperature history of the thermal storage material composition 1, showing the process of natural cooling of the sample immersed in the foam, during which the process of crystallization of materials occurs. This process showed that the substances begin to solidify at t=34.94°C, the supercooling is 2.87°C, and the accumulation time is 45 minutes. At all stages of the study of the substance, no water was released, the mixture crystallized completely.

In FIG. 3 shows the dependence of the viscosity of this composition on the rotation of the metal rod. As the graph reaches a plateau, it can be seen that the dynamic viscosity is 1066 mPa⋅s. The dynamic viscosity and density of the heat storage material of composition 1 are presented in Table 1. The isobaric heat capacity is calculated for the liquid phase.

Composition 2:

(70% Ca (NO ₃ ) ₂ ⋅4H ₂ O + 30% Cd (NO ₃ ) ₂ ⋅ 4H ₂ O) + 1% CaO + 1% CMC

Crystalline hydrates of calcium and cadmium nitrates weighing 14 and 6 g (2 identical mixtures were prepared), respectively, were weighed in a ratio of 70:30 and heated to 60°C, after which they were melted for 30 minutes with constant stirring until complete melting, and then kept in within 10 minutes. Then, for efficient nucleation and prevention of phase segregation, 1% CaO and carboxymethyl cellulose (CMC) (0.2 g each) were added. All ratios, with the exception of CMC, are taken by weight. Carboxymethylcellulose as a thickener is taken in volume fraction. Within three hours after the addition, the mixture was stirred at a maximum heating temperature of 60°C.

To confirm the properties of the synthesized materials by differential scanning calorimetry (DSC), the temperature and heat of melting, as well as the magnitude of the temperature hysteresis, were studied. Experimental conditions by DSC method:

• Maximum heating temperature, °С, 70;

• Minimum cooling temperature, °С, 0;

• Heating rate, °С/min: 10;

• Atmosphere, N ₂ ;

• Cooling rate, °С/min: 2;

• Gas for cooling, N ₂ ;

• Gas supply rate ml/min, 40.

In FIG. Figure 4 shows the DSC plot of a mixture of composition 2 with a melting heat of 112.3 J/g and a phase transition onset temperature of 39.80°C.

In FIG. 5 is a graph of the temperature history of the composition 2 thermal storage material showing the natural cooling process in which the material crystallizes. This process showed that the mixture begins to solidify at t=25.87°C, the subcooling is 0.44°C, and the accumulation time is 30 minutes.

At all stages of the study of the substance, no water was released, the material crystallized completely.

In FIG. 6 shows the dependence of viscosity on the rotation of a metal rod. As the graph reaches a plateau, it can be seen that the dynamic viscosity is 1690 mPa⋅s. The dynamic viscosity and density of the heat storage material of composition 2 are presented in Table 1. The isobaric heat capacity is calculated for the liquid phase.

Composition 3:

(70% Ca (NO ₃ ) ₂ ⋅4H ₂ O + 30% Cd (NO ₃ ) ₂ ⋅ 4H ₂ O) + 1% Ca (OH) ₂ + 1% CMC

Crystalline hydrates of calcium and cadmium nitrates weighing 14 and 6 g (2 identical mixtures were prepared), respectively, were weighed in a ratio of 70:30 and heated to 60°C, after which they were melted for 30 minutes with constant stirring until complete melting, and then kept in within 10 minutes. Then, for effective nucleation and prevention of phase segregation, 1% Ca(OH) was added.₂ and carboxymethylcellulose (CMC) (0.2 g each). All ratios, with the exception of CMC, are taken by weight. Carboxymethylcellulose as a thickener is taken in volume fraction. Within three hours after the addition, the mixture was stirred at a maximum heating temperature of 60°C.

To confirm the properties of the synthesized materials by differential scanning calorimetry (DSC), the temperature and heat of melting, as well as the magnitude of the temperature hysteresis, were studied. Experimental conditions by DSC method:

• Maximum heating temperature, °С, 70;

• Minimum cooling temperature, °С, 0;

• Heating rate, °С/min: 10;

• Atmosphere, N ₂ ;

• Cooling rate, °С/min: 2;

• Gas for cooling, N ₂ ;

• Gas supply rate ml/min, 40.

In FIG. Figure 7 shows the DSC plot of a material with a melting heat of 120.8 J/g and a phase transition onset temperature of 42.19°C.

In FIG. 8 is a graph of the temperature history of the composition 3 thermal storage material showing the natural cooling process in which the material crystallizes. This process showed that the mixture begins to solidify at t=24.81°C, there is no supercooling, and the accumulation time is 66 minutes.

At all stages of the study of the substance, no water was released, the material crystallized completely.

In FIG. 9 shows the dependence of viscosity on the rotation of a metal rod. As the graph reaches a plateau, it can be seen that the dynamic viscosity is 2724 mPa⋅s. The dynamic viscosity and density of the heat storage material of composition 3 are presented in Table 1. The isobaric heat capacity is calculated for the liquid phase.

The general physicochemical characteristics of the synthesized compositions are shown in Table 1.

Samples of substances weighing 15-20 grams, studied by the method of temperature history, more accurately show the crystallization temperature of the material in practical operation (Safari A., Saidur R., Sulaiman FA, Xu Y., Dong J. A review on supercooling of Phase Change Materials in thermal energy storage systems // Renewable & Sustainable Energy Reviews. - 2017. - T. 70. - C. 905-919).

Thus, the properties of heat storage compositions based on a eutectic mixture of calcium and cadmium nitrate crystalline hydrates have low supercooling, ranging from 2.87°C to 0°C, depending on the type of additive. The melting enthalpy of the compositions varies in the temperature range from 112.3 to 124.9 J/g, however, due to the high densities of the liquid phase of the materials, the accumulation density ranges from 214.4 to 406.0 MJ/m ³ . At the same time, the accumulation time of the material varies, depending on the composition of the additives, from 30 to 66 minutes, the crystallization onset temperature is from 24.81 to 34.94°C, and the melting point is from 39.80 to 43.42°C.

Based on these data, it can be judged that heat storage compositions can be used for space heating in the temperature range from 36 to 45°C. Composition 1 crystallizes at 37.81°C and melts at 43.42°C, which indicates a low temperature hysteresis. The heat storage density of composition 1 reaches 214.4 J/g. The heat storage materials of compositions 2 and 3 crystallize at 26.31 and 24.81°C and melt at 39.80 and 42.19°C, respectively. Despite the relatively high temperature hysteresis, the present compositions are suitable as TAM due to the high density heat storage, reaching 292.0 and 406.0 MJ / m ^3, respectively.

Claims (1)

Heat storage composition with a phase transition based on a eutectic mixture of calcium nitrate tetrahydrate and cadmium nitrate tetrahydrate, containing a eutectic mixture of Ca(NO ₃ ) ₂ ⋅4Н ₂ O and Cd(NO ₃ ) ₂ ⋅4H ₂ O with the addition of expanded graphite and carboxymethyl cellulose, or eutectic a mixture of Ca(NO ₃ ) ₂ ⋅4H ₂ O and Cd(NO ₃ ) ₂ ⋅4H ₂ O with additions of calcium oxide and carboxymethyl cellulose, or a eutectic mixture of Ca(NO ₃ ) ₂ ⋅4H ₂ O and Cd(NO ₃ ) ₂ ⋅ 4H ₂ O with additions of calcium hydroxide and carboxymethylcellulose, characterized by a phase transition temperature between 36 and 45°C, which ensures the performance of the composition as a heat storage material, prepared by heating to 60°C a eutectic mixture of crystalline hydrates of calcium nitrate tetrahydrate Ca(NO ₃ ) ₂ ⋅ 4H ₂ O and cadmium nitrate tetrahydrate Cd(NO ₃ ) ₂ ⋅4H ₂ O and melting for 30 min with constant stirring until complete melting, holding for 10 min, sequential addition of additives and further stirring for three hours with a temperature control of 60°C.

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