Determination of components for heat storage material

Today, the world is transitioning to the 4th and 5th Generation District Heating (GDH) systems of heating networks. The main advantages of 4GDH and 5GDH are the reduction of the temperature of the coolant to +50 – +60°C and the use of various designs and operating principles of thermal energy storage. Therefore, special attention is devoted to the search for new types of coolants, heat-accumulating materials and research of their properties and determination of optimal operating modes. One of the energy storage systems that has become widespread in recent years is the mobile thermal energy storage (M-TES) that has tanks filled with heat storage material. Specially created heat storage materials are often used, which work under the necessary technological conditions and meet the strict conditions of safe transportation of liquids. This article is devoted to the search for new components for the creation of heat storage material, which will be used in capacitor-type TES together with the “thermal core”, which is created from a material with a phase transition. As a result of experimental studies of the thermocycling process of water and aqueous solutions, depending on the components added, the priority components such as guar gum and xanthan gum were chosen.


Introduction
The rapid growth of energy consumption in industry and by the population has changed the requirements approaches to heat and power supply.To meet the ever-increasing demand, advanced economies are using more energy efficient 4GDH and 5GDH generation heating systems.Their main features are the environmental friendliness and economic expediency of the use of resources, the minimization of heat losses during heat supply, ensuring the stability of the energy and heat systems [1,2].The main advantages of 4GDH and 5GDH systems are: • lowering the temperature of the coolant to +50 -+60℃, which saves energy costs for its heating; • the use of various energy sources with a wide involvement of renewable generation sources operating with the use of thermal energy storages (TES), which makes it possible to stabilize the heat supply system and equalize peak loads [3,4]; • the use of main pipelines to supply heat to the consumer in winter, as well as cold for the central air conditioning system in summer; • high degree of automation and dispatching of process control.
One of the main elements in many links of energy supply are heat carriers and heat storage materials [5].Nowadays, researchers pay great attention to the study of the thermal properties of common liquid heat carriers, heat storage materials and materials with a phase transition [6,7] and conduct active research to find new compositions as well [8,9].
The most common coolant is water, since it has an abnormally high value of specific heat capacity of 4.2 kJ/(kg•K).This explains its ability to quickly heat up and cool down, respectively, the TES has a fast charge and discharge time.However, the main task of the TES is a long time of heat accumulation.To extend the accumulation time, multifunctional powders are used that are capable of forming water-soluble polymers (WRPs).Multifunctional powders due to their various chemical, physical and thermophysical properties are used in pharmaceutical, food, mining, paper textile, industries, as well as in construction, agriculture, oil production, etc.For research, the following substances were selected: carboxymethylcellulose, guar and xanthan gums, from which solutions were prepared with mass concentrations of 1% and 10%.The advantage is that they are able to form solutions with stable physicochemical parameters during thermal cycling.These solutions are characterized by low corrosivity, resistant to crystallization, economically viable, environmentally friendly and fire and explosion safe.
Liu et al [10,11] propose a new approach to the heat supply system by creating M-TES, which will allow organizing a mobile heat supply process for enterprises and public utilities.Mobile heat supply is of particular importance during the war in Ukraine.The main element is a capacitive-type TES filled with accumulative substances for low-potential heating systems.For this, a test bench was created to study the process of thermocycling of heat carriers and heat storage materials.
The purpose of the research is to determine the possibility of using some components to create a heat storage material by thermocycling their aqueous solutions with subsequent use in heat storages and heat supply systems with an operating temperature range from 50℃ to 120℃.

Materials and methods
The choice of components for creating heat storage material was carried out taking into account the technological parameters and the specifics of its use in mobile thermal energy storage, namely: • the rate of discharge and charge of thermal storage tanks M-TES; • longer accumulation time; • explosion and fire safety during transportation of M-TES; • availability and economic feasibility of the used components.
Taking into account the specified technological requirements, some water-soluble polymers were selected for research, which we will consider in more detail [11,12].

Materials
Carboxymethylcellulose [13,14] (CMC, food additive E466) is obtained by processing cellulose of the general composition [C 6 H 7 O 2 (OH) 3−x (OCH 2 COOH) x ] n , where x = 0.08 − 1.5 (figure 1).CMC (LLC "Khimpostachannya", China) is a substance with amorphous properties that acts as a weak acid.By its chemical nature, it is a highly polymeric ionic electrolyte.
CMC is a colorless substance that dissolves in water, has no odor and is safe to use.CMC is insoluble in vegetable oils and animal fats, does not decompose under the action of sunlight.But it dissolves in organic acids: formic, lactic and glacial acetic.
To obtain a homogeneous CMC solution, dry substances are soaked in water at a temperature of 80-85°C, then cold water is added.With an increase in temperature, the viscosity of CMC solutions decreases or phase separation or gel formation occurs.Xanthan gum [15,16] (C 35 H 49 O 29 ) n is a natural chemical compound that is used primarily as a food additive E415 and belongs to the group of stabilizers.Xanthan gum (LLC "Khimpostachannya", China) is a linear hydrocarbon polysaccharide, the molecular weight and properties of which can be controlled by changing the living conditions of Xanthomonascampestris microorganisms (figure 2).Xanthan solution does not interact with alkalis and acids (except hydrochloric), alcohols and enzymes, and is also stable in the temperature range from 120 to 18°C.The water-soluble polymer (WRP) based on xanthan gum is characterized by high viscosity in the pH range from 2 to 12, therefore, a WRP with a dense structure is formed, which stabilizes foodstuffs for a long time and prolongs their shelf life.
Guar gum (C 6 H 10 O 5 ) n [17] is a white or yellowish powder with a characteristic odour.It is obtained from ground seeds of guar beans containing up to 70% gum (figure 3).Guar gum (TM "Zhyvy Zdorovo", Ukraine) is used as a thickening agent or stabilizer.The main advantage of natural plant polysaccharides is low cost, but their technological indicators are low, which narrows the scope of application.Therefore, chemically modified derivatives of guar, cellulose and starch are used, which have the necessary technological properties.
In the early 1970s, hydroxypropyl guar (HPG) was obtained, which became the most widely used thickener for process fluids.HPG makes it possible to obtain a polymer that is more viscous and resistant to high temperatures.Guar products typically contain 8 -12% non-hydrated residue, HPG residue is 1 -4% In some experiments, a solution of sodium bicarbonate (N aHCO 3 ) with a mass concentration of 10% was used.In addition, antifreeze "DEFREEZE" (JSC "Bishofit", Ukraine) was added to the composition of the heat storage material."DEFREEZE" is a non-toxic light yellow liquid, consisting of distilled water, natural magnesium hexahydrate, organic stabilizers and corrosion inhibitors.It does not contain synthetic and toxic substances, including ethylene, alcohols, amines, nitrites.

Test bench
The study of the thermocycling process of heat storage liquid was carried out on an experimental bench, the description of which is presented in [18].The stand in a simplified form simulates the process that occurs in the heat storage capacity of M-TES [19].The test bench can be represented as two containers installed one inside the other (figure 4).
A heat storage of the capacitive type has two metal containers: • external tank 1 filled with heat carrier (or heat storage material); • tank 2, which is installed in the centre of tank 1, is filled with phase change material (PCM) and hermetically closed, similarly to the M-TES design.Tank 2 is called the "thermal core" and provides an increase in the heat storage capacity of the tank 1, equalizes the temperature field in the tank volume, reduces stratification by the height of the battery, shortens the "charging" time and extends the "discharging" time.
Ceresin was used to fill the "thermal core".Ceresin is PCM of natural origin, with a phase transition in the temperature range of 61 -78°C, which coincides with the working temperatures of M-TES.Preliminary studies of specific heat capacity changes before and after thermocycling proved the feasibility of using this material [20].Substances in tank 1 and 2 were heated by an electric heater 3 installed in the lower part of tank 1 (or an electric device in the case of an experimental stand).
The temperature was measured with thermocouples 4 using a microprocessor module "TRITON 6004TS" 5 (Scientific and Production Private Enterprise "TEREKS", Ukraine).The temperature was recorded using thermocouples and automatic cold junction temperature compensation.The main characteristics of the microprocessor module are shown in table 1 Visualization of experimental studies was carried out using the Data Recorder software installed on a PC.Heat consumption was measured with an electric energy meter 6.

Research methodology
The studied substances in the specified combinations (table 2) are placed in tanks 1 and 2. In tank 1 samples of heat storage material with a volume of 1 liter are examined.Tank 2 contains PCM material with a volume of up to 100 ml, which was specially selected experimentally [20].
Tank 2 is the "thermal core".The volumes of the studied substances were selected in a ratio of 10:1 respectively.The containers are hermetically sealed with lids, on which are pre-installed thermocouples: t 1 is the ambient temperature; t 2 is the temperature of the water system in tank 1; t 3 is the temperature of the substance in tank 2.
Using the Data Recorder program, the visualization of the beginning of temperature measurements is observed.Meter 6 records the energy indicators.We record the indicators of the microprocessor module 5 and the heat meter 6. Heating of the studied substances carried out in the temperature range similar to the temperature load of M-TES.At the temperature of t 2 = 85℃, the heat supply is turned off and the temperature and power consumption continued to be measured.At the temperature t 2 = 30℃, the experiment is completed.
When conducting research, the following parameters are measured: • temperature changes in the heat storage material in the tank 1, PCM in the tank 2 and the temperature of the surrounding air; • heating and cooling time, which will allow to determine the charging and discharging time of the TES; • energy consumption spent on heating heat storage material and material with a phase transition to the specified temperatures.
Each experiment was repeated four times.Graphical dependencies, which are based on the averaged data, are presented below.
Substances that can be added as components of the future heat-accumulating material were selected for study.The combination of substances in the tank 1 and the tank 2 are shown in table 2. The serial number of each experiment corresponds to the numbering in the figures presented in section 3 of this article.

Results
With the help of the "TRITON 6004TS" microprocessor module, graphical dependences of the temperature changes of the samples over time were obtained.Dependencies are built on their basis, which make it possible to evaluate the effectiveness of the components.Experiment No. 1 is chosen as a "reference" because instead of a heat storage material, water is used, the thermophysical characteristics of which are known.We will conduct a comparison relative to its results.
The energy consumption estimate for each of the experiments is shown in figure 5.
The maximum energy consumption per cycle was recorded in experiment No. 1, where water heats the PCM.The lowest energy consumption per cycle was observed in Experiment No. 10, where the heat storage material includes antifreeze and xanthan gum in the amount of 1%.The difference between these experiments is 27.3%, which is a significant difference for the average consumer and unacceptable for industry.
In experiments No. 4, No. 5 and No. 9, the energy consumption for 1 cycle is 21.5% less than in experiment No. 1 that confirms the effectiveness of using the CMC component, xanthan  The duration of one cycle for each experiment conducted in test bench is presented in figure 6.The experiment time includes the heating time and the cooling time, which is equal to one cycle.It can be seen in figure 6 that the shortest cycle time was in experiment No. 1 where the water heated the PCM.This can be explained by the abnormally high specific heat capacity compared to any other liquid.In experiment No. 2, water was replaced with a heat storage material made of water and antifreeze, which extended the cycle time by 15%.But the longest cycle time is in experiment No. 7, which is 24% longer than experiment No 1, which indicates an extended period of accumulation due to the addition of 1% guar gum.Also, it is necessary to note experiments No. 4, 8, in which the cycle time is extended by 14.7% and 13.8%, respectively.But during the thermocycling process in experiment No. 4, foaming, an increase in the volume of heat storage material and a sharp smell were observed, which makes it impossible to use soda as a component.In experiments No 9 and No 10, the cycle time was increased by 9.5% and 11.4% respectively.
The ratio of the spent energy to the time of the "heating-cooling" cycle is presented in figure 7.
The ratio of the spent energy to the time of the "heating-cooling" cycle allows to assess the efficiency of the heat storage material.Therefore, experiment No. 1 is the least effective, as it   The composition of the substance in experiment No. 2, which shows a difference from experiment No. 1 by 25.6%, is, as a rule, effective when used in heating systems of private houses.Experiments No. 4, No. 5 and No. 9 have similar results and are 31.6%,29.5%, and 28.3%, respectively.
The dynamics of changes in the mass of PCM materials at the beginning of the research and after 40 cycles of thermocycling are shown in figure 8.
The PCM sample, in this case ceresin, was filled into container 2 and did not change throughout the entire cycle of experiments from experiment No. 1 to experiment No. 10.Weight measurements were carried out before and after thermal cycling.Samples were weighed on an Ohaus AX224 (USA) electronic balance (220g/0.1mg)with automatic calibration.The initial mass before Experiment No. 1 was 107.53 g, after the last Experiment No. 10, the mass was equal to 106.2 g.This means that in the process of 40 cycles of thermocycling (heatingcooling) the mass changed within 1 -2% and this material can be used to fill the "thermal core" of M-TES.Such minor changes in mass, as well as the absence of volume expansion observed during the research, testify to the safety and reliability of this PCM.

Conclusions
The conducted experimental studies made it possible to draw the following conclusions: • heat storage material, in addition to water and antifreeze, must contain guar gum, as they allow to create a substance that significantly extends the term of heat storage compared to water by 15 -24%; • the lowest energy consumption per cycle was noted with the addition of xanthan gum, this indicator decreases relative to the energy consumption of water by 27.3%.When the ratio of spent energy to the time of the thermocycling cycle is maintained, this trend is maintained and amounts to 34.8%; • during long-term thermocycling of ceresin for up to 40 cycles, which fills the "thermal core" in the M-TES design, changes in the mass of PCM occur up to 2%, which indicates the safety and reliability of its use.
As a result of the conducted experimental studies, the expediency of using water, antifreeze, xanthan and guar gum as part of the heat storage material was established.Determining the concentrations of these substances will be the subjects of further research.
The safety of using ceresin in the "thermal core" was also confirmed by studying the change in mass before and after thermosetting.

Figure 6 .
Figure 6.Average period of time per cycle.

Figure 7 .
Figure 7. Ratio of energy consumption from time.

Figure 8 .
Figure 8. Dynamics of change in the weight of PCM materials (row 1 -initial mass of PCM, row 2 -final mass of PCM (after thermocycling)).

Table 2 .
Order of heat storage materials combination when conducting experimental studies.
requires a significant supply of energy with a minimum accumulation time.Experiment No. 10 is the most effective, as it has the lowest value of the ratio, in general it is 34.8%.That is, when conducting further research, xanthan gum is one of the priority components. 10