High-temperature Thermal Properties and Wear Behavior of Basalt as Heat Storage Material for Concentrated Solar Power Plants

LIAO Jun, ZHU Xupeng, LI Jianan, XUE Shuwen, ZOU Changwei, ZHANG Jun

(School of Physics Science and Technology, Lingnan Normal University, Zhanjiang 524048, China)

Abstract: The microstructures, components, thermal stability, specific heat capacity and thermal conductivity of basalt sample were studied. Besides, as a comprehensive result of thermal expansion and contraction process, both the friction coefficient and wear rate of the basalt sample were also characterized. Our results indicate that basalt is an excellent candidate to be used as thermal energy storage material for concentrated solar power plants, and also provide a strategy for solar energy utilization in volcanic area with excellent geographical environment.

Key words: thermal energy storage; concentrated solar power; basalt; wear rate

1 Introduction

As non-renewable resources such as coal are gradually depleted and the greatly increased demand for energy consumption, the problem of global energy shortage has caused widespread concern in society. Various forms of renewable energy utilization and energy storage have been continuously developed and applied, such as solar power generation, solar heat collection, wind power generation, hydropower generation, various types of power storageand heat storage devices,

etc

. to solve this problem. Among them, solar energy has been widely concerned and used because it is clean, renewable and can be used directly. However, the utilization efficiency of solar energy is affected by geographical environment and the changes of day and night. Therefore, how to further improve the utilization efficiency of solar energy is an important issue worthy studying. Concentrated solar power (CSP) is a non-polluting and high-efficiency solar utilization method. The accompanying concentrated solar heat storage system can store affluent solar energy resources into the heat energy storage materials and release it to generate electricity at night or without direct sunlight, which can further improve the efficiency of solar energy, so it is a very promising solution to the world’s energy problems.

The solar thermal power generation system in the CSP plants includes point-focused solar power generation (dish type), line-focused solar power generation (trough type) and surface-focused solar power generation (tower type). The latter two are widely used, the operating temperature is generally 20-565 ℃, and the heat storage medium is usually water, thermal oil or molten salt. Molten salt is the most widely used heat storage medium due to the higher operating temperature, but the high operating cost caused by the higher freezing point of most molten salts limits the application of this medium system. In addition, centralized solar heat storage systems in CSP plants require tens of thousands of tons of heat storage materials to store the heat required for power generation at night or without sun exposure, so there is an urgent need to find efficient and economical heat storage materials. In thermal storage applications, thermal storage materials should first have the characteristics of high heat capacity and high thermal conductivity, and secondly, they should be abundant, cheap and have an excellent mechanical stability. Based on this index, the use of volcanic rocks (such as basalt, which can be used as a heat storage material for affluent solar energy of the accompanying concentrated solar heat storage system) formed at ultra-high temperatures during volcanic eruption as heat storage materials can increase the solar energy utilization rate in CSP plants and reduce their operating costs. Although the heat storage properties of basalt have been reported by some researchers, the study on changes of high-temperature performance such as the microstructures, components, thermal stability, specific heat capacity and thermal conductivity of basalt during the heat storage process still needs further research. Besides, as a heat storage material in a packed bed arrangement,which can be integrated with the heat transfer medium system of any types of CSP system, basalt have to undergo high and low temperature environment changes during the storing and releasing process, the mechanical wear of basalt caused by the thermal expansion and contraction effect may directly decompose the stone body, therefore the mechanical stability of basalt also needs to be studied.

In this work, we aimed to study the feasibility of basalt as a backup heat storage material in CSP plants. The microstructures, components, thermal stability, specific heat capacity and thermal conductivity of basalt sample during the storing and releasing process were systematically studied. Besides, both the wear rate and friction coefficient of the basalt sample were also be characterized. The results revealed that basalt is a potential heat storage material. Our work provides possible strategies for solar energy utilization in volcanic area with excellent geographical environment and indicates that basalt is an excellent candidate to be used as heat storage material in CSP plants.

2 Experimental

2.1 Materials

Basalt is a kind of rock with dense or foam-like structures, which is formed by cooling and solidifying lava from volcanic eruption, and also is an important component of the earth’s oceanic crust and continental crust. Basalt belongs to the basic extrusive rock, the color is usually black, sometimes grayish green or dark purple, and it has the characteristics of high temperature resistance, high density and high wear resistance properties due to its extreme formation environment. Therefore, basalt is a kind of potential natural high temperature heat storage material. The basalt sample studied in this work was gathered from Leizhou Peninsula (Zhanjiang, China), as shown in Fig.1. Zhanjiang has one of China’s large volcanic clusters, the Leizhou Peninsula volcanic group, with a basalt platform area of 3 898 kmand 76 volcanoes, so it has extremely rich basalt resources. At the same time, Zhanjiang has an excellent geographical and climatic conditions due to the low temperature difference between day and night. According to the statistics in the period of 2009-2018, the difference between the highest and lowest monthly temperatures is about 6 ℃, and the difference between the highest and lowest temperatures in a year does not exceed 19 ℃. Besides, the average annual total solar radiation can be as high as 4 265.29 MJ/(m·d), and the average daily solar irradiance in the four seasons is between 8.8-14.2 MJ/(m·d), which belongs to the general area of solar energy resources, so further improving the utilization efficiency of solar energy will play a positive role in the economic development of Zhanjiang, China. In the case of quantitative solar resources, the low temperature difference can reduce the maintenance cost of the heat storage system and additional heat loss, and the huge basalt reserves reduce the transportation and material cost of the heat storage materials required by the CSP plants in Zhanjiang. Therefore, it is very necessary to make an in-depth study on the solar thermal storage performance and wear of basalt in Leizhou Peninsula.

Fig.1 The picture of basalt

2.2 Micromorphology, composition analysis and thermal stability

The micromorphology of the basalt sample before and after 3 thermogravimetric analysis (TGA) were characterized by a JSM-7610F scanning electron microscope (SEM). The elemental composition analysis before and after 3 annealing treatments (heating from room temperature (RT) to 850 ℃ with a heat rate of 20 ℃/min) were assessed by energy dispersive spectrometer (EDS) equipped in SEM. It should be noted that we powdered the basalt sample to reduce the differences in the composition of different characterization selections before the EDS test. The composition analysis of both basalt powder samples before and after 3 annealing treatments were characterized by X-ray diffraction (XRD, PANalytical Xpert pro MRD diffractometer) by placing basalt powders on a glass slide. The data collection range is 10°-80° and the step size is 0.02°.

The thermal stability and TGA of basalt sample before and after 10 high-temperature shocks (annealing treatments, heating from RT to 850 ℃ with a heat rate of 20 ℃/min) were characterized by Perkinelmer STA6000 simultaneous thermal analyzer. The specific operation process of TGA is completed in a nitrogen atmosphere, and the temperature heating from RT to 860 ℃ with a heat rate of 20 ℃/min. The mass of the basalt sample used for this testing before and after 10 high-temperature shocks is 33.258 and 15.969 mg, respectively. The material thermal stability of the basalt sample was determined by 3 TGA between 60 and 860 ℃.

2.3 Specific heat, thermal conductivity, and thermal diffusivity

The specific heat capacity of the basalt sample at different temperatures were obtained from differential scanning calorimetry (DSC) curves which carried out on a DSC-200F3 thermal analyzer under nitrogen atmosphere. A sapphire was used as a reference material in instrument. The thermal diffusivity of the basalt sample at different temperatures were measured by laser flash analysis (LFA) 427 from Netzsch and the test temperature RT, 100, 300, and 500 ℃. The basalt sample was prepared in a circular form with a diameter of 12.7 mm and a thickness of 2 mm before the thermal diffusivity test. The thermal conductivity of the basalt sample at different temperatures were calculated by:

where, α represents the thermal diffusivity of basalt sample, ρ represents the density of basalt sample, and

C

represents the specific heat capacity of basalt sample. The density data was averaged of four measurements (mass/volume) for samples with different size to assure the accuracy of the result.

2.4 Wear test

A comprehensive tester for material surface performance (CFT-1) were used to conduct wear test by rotating a large disc basalt against sliding dry pin basalt in the air. The contact surface is a circle with a radius of 4.5 mm. During the test, the friction coefficient is obtained by the computer continuously calculating the data recorded by the detector. The running radius of 6 mm was selected, the running speed is 200 r/min, the applied load is 500 g and the duration time is 120 min. An electronic scale was used to measure the mass of pin basalt and disc basalt before and after wear test, and the related measurement accuracy is g/1000. The wear rate of basalt was calculated by:

where,

M

represents the difference in basalt weight before and after wear test;

F

and

S

represents the sliding distance and the applied load, respectively.

3 Results and discussion

3.1 Micromorphology and composition

Fig.2 shows the micro-surface features of the test basalt sample before (Figs.2(a)-2(c)) and after (Figs.2(e)-2(g)) 3 TGA. The magnification of SEM pictures was 40, 500, and 1 000 times (×40, ×500, and ×1 000) in sequence. The coarse pores can be observed on both the surfaces of test basalt before and after thermal treatment (Figs.2(a) and 2(e)). The pores were formed when magma cooled down in air condition. The surfaces of test basalt before and after thermal treatment showed typical detrital structures in dense areas. The detrital structures of the basalt sample after heat treatment are more obvious, the reason might be attributed to the rougher sample cutting method. However, in a smaller area (Figs.2(c) and 2(g)), both the morphology before and after heat treatment is a typical exfoliated structure determined by the essence of mechanical interaction between processing tool and basalt sample, which indicates that the micromorphology of the basalt sample remains basically unchanged under multiple thermal shocks.

Fig.2 SEM images of studied basalt sample before and after 3 consecutive TGA

Fig.3 SEM images and the EDS spectra of studied basalt sample before and after three annealing treatments

To make the composition of the basalt sample more clear and reduce the differences in the composition of different characterization selections, we pulverized a new sample and annealed it three times (heating from RT to 850 ℃ with heat rate of 20 ℃/min, which is basically the same as the temperature change during the TGA). Figs.3(a)-3(b) and Figs.3(c)-3(d) show the selection of EDS test and the related EDS spectrum of the basalt sample before and after three annealing treatments, respectively. The results indicate that the basalt sample is composed of oxygen, silicon, iron, calcium, aluminum, carbon, potassium, sodium, magnesium and titanium,

etc

. Oxygen and silicon are the two most abundant elements in basalt sample. Calcium, aluminum and iron are also important components of test basalt sample. Through the comparison of the element composition and content before and after heat treatment, it is not difficult to find that there is no obvious element loss or increase process in the process of multiple annealing treatments, indicating that the element composition of the sample is basically stable during the thermal cycle treatments process.The material composition of test basalt sample before and after 3 annealing treatments were characterized by the corresponding XRD characteristics, as shown in Fig.4. The result before annealing treatments indicates that the XRD characteristics corresponding to the main components of the test basalt sample are consistent with the main components in the labradorite (marked by blue circles, PDF Number:83-1368) and diopside ferrian (marked by blue dots, PDF Number: 89-834). At the same time, the XRD characteristics of epsilon-iron oxide (

ε

-FeO, PDF Number: 16-895) and magnesium oxide (MgO, PDF Number: 2-1207) have also been found in the test basalt sample. After 3 annealing treatments, the XRD characteristics of all components of basalt before annealing treatments can be found in XRD curve of test basalt after annealing treatments, which further shows that there is no obvious composition change during the annealing process. It should be noted that in order to make the XRD features of basalt before annealing treatments more obvious, we have performed a three-fold magnification process on its XRD curve.

Fig.4 XRD patterns of test basalt sample before and after three annealing treatments

3.2 Thermal stability

To qualitatively describe the thermal stability, 3 consecutive TGA of basalt samples were carried out under the high-purity nitrogen atmosphere before and after 10 high-temperature shocks. The operating temperature range is 60-860 ℃ and the related TGA carves of basalt sample as shown in Fig.5. The results in Fig.5(a) indicate that the mass of basalt has a slight decrease with rising temperature in the first thermal cycle before 10 high-temperature shocks. The mass loss, about 1.25%, might be caused by the evaporation of water. When the second and third thermal cycles were carried out, the mass of the basalt sample basically no longer changes, indicating that the basalt remains stable under high-temperature shocks up to 860 ℃. After 10 high-temperature shocks (Fig.5(b)), the mass of basalt is almost not lost with rising temperature in all of thermal cycle. This result further shows that basalt has good thermal stability. It worth noting that in the subsequent thermogravimetric test, the slight downward shift of the mass loss curves is likely to be caused by the instrument errors.

Fig.5 TGA curves of studied basalt sample before and after 10 high-temperature shocks

3.3 Specific heat capacity, thermal conductivity, and thermal diffusivity

Fig.6(a) displays the specific heat capacity of basalt samples with temperature from RT to 500 ℃. The average density of the basalt sample measured at RT is 2 393 kg/m. The thermal capacity is a function of temperature (specific heat capacity × density) due to the usually fixed density constant with rising temperature. The results show that the specific heat capacity of basalt generally increases with the increase of temperature, and its change trend is consistent with the literature on heat storage materials. From RT to 500 ℃, the specific heat capacity of test basalt sample increased from 0.827 to 1.075 J/(g·K), and the value at RT is within the range summarized by Tiskatine

et al

. Fig.6(b) shows the thermal conductivity (red part) and thermal diffusivity (blue part) of basalt sample with temperature from RT to 500 ℃. The thermal diffusivity of basalt sample decreased from 1.133 to 0.894 mm/s with temperature changed from RT to 500 ℃. The thermal conductivity of basalt sample at different temperatures is in the range of 1.9-2.3 W/(m·K). The specific value is 2.242 W/(m·K) at RT and 2.3 W/(m·K) at 500 ℃, respectively, which is also within the range reported in literature at RT. The specific heat capacity and thermal conductivity of a serious of thermal energy storage materials used in CPS plants as shown in Table 1. Comparing with the published data on sensible thermal energy storage materials, basalt shows good thermal properties in high temperature zone.

Fig.6 (a) Specific heat capacity of basalt sample at different selected temperatures; (b) Thermal conductivity and thermal diffusivity of basalt sample at different selected temperatures

Table 1 Performance comparison of known commonly used heat storage materials

Materials ρ /(kg/m3)Cρ /(J/g·K)λ/(W/m·K)Castable ceramic (350 ℃) 3 500 0.866 1.35 Silicone oil (400 ℃) 900 2.1 0.1 Reinforced concrete (400 ℃) 2 200 0.85 1.5 Nitrite salts (450 ℃) 1 825 1.5 0.57 s slag (500 ℃) 4 260 0.858 1.736 c slag (500 ℃) 3 610 0.975 1.75 Basalts (500 ℃) 2 393 1.075 2.3

3.4 Wear behavior

The friction and wear behavior of basalt sample before and after 10 high-temperature shocks were characterized by a comprehensive tester for material surface performance (Fig.7(a)). The entire system of control and record is mainly composed of three parts: force detector, speed detector and drive controller. As show in Fig.7(b), the friction coefficient of basalt sample is in the range of 0.9-1.1 before 10 high-temperature shocks and the friction coefficient of basalt sample increases a litter after 10 high-temperature shocks. There were some sudden changes in the friction coefficient during the test due to the presence of coarse pores in test sample. The wear rate of basalt sample before and after 10 high-temperature shocks were calculated by Eq.(2). After the detailed calculation, we got the average wear rate of basalt samples is 5.41×10and 4.81×10g/(N·m), respectively, before and after 10 high-temperature shocks. This value can be compared with the value of heat storage materials prepared from slags. It should be noted that the small difference of friction coefficient and average wear rate of basalt before and after 10 high-temperature shocks is probably caused by the different samples selected. The above results show excellent friction properties of basalt and further indicate basalts have a good stability, which is a potential heat storage material in CSP plant.

Fig.7 (a) The schematic diagram of the friction and wear test; (b) The friction coefficient and average wear rate of basalt sample before and after 10 high-temperature shocks

4 Conclusions

In this work, both the thermal characteristics and wear behaviors of basalt sample in Leizhou Peninsula (China) were studied. The obtained results revealed: basalt is stable up to 860 ℃. Basalt have a high heat capacity and high thermal conductivity. The experimental specific heat capacity increased with temperature from 0.827 J/(g·K) at RT to 1.075 J/(g·K) at 500 ℃. The thermal conductivity value is 2.242 W/(m·K) at RT and 2.3 W/(m·K) at 500 ℃. Basalt exhibit excellent wear resistant, indicating basalts are suitable in thermal energy storage system. Therefore, the advantages of thermal characteristics and friction properties make basalt is a highly potential heat storage material for thermal energy storage system in CSP plants. Our results provide a strategy for solar energy utilization in CSP plants.