Dataset on thermophysical properties of natural stones for heat storage applications

Abstract

The need for thermal energy storage technologies is critical because they can play a key role in enabling renewable heat systems to be adopted for industrial applications and reduce consumption of fossil fuels. Earlier studies have indicated that, natural rocks have potential for cost effective heat storage applications and in this data set, thermophysical properties of natural rock samples including, (basalt, dolerite, gabbro, granite, rhyolite, gneiss and quartzite) from Zimbabwe, were determined. Both experimental methods and numerical calculations were applied to generate this data. Specific heat capacity, thermal stability and density were obtained using experimental measurements while thermal diffusivity was determined through calculations using data from experiments and literature. Values of specific heat capacity for all rock samples, as obtained from the Differential scanning calorimetry, range from 767 J/kgK and 861 J/kgK to 942 J/kgK and 1090 J/kgK, at room temperature and at 250 ℃ respectively. Thermogravimetric analysis for thermal stability measurements of rocks produced a data set which indicates that, the samples have a maximum weight loss of less than 5 % when heated up to temperatures of 700 ℃. Experimental measurements for wet density of all rock samples have shown that the values vary between 2500 kg/m³ and 3001 kg/m³. Deduced values of thermal diffusivity vary from 2.14 mm²/s and 0.79 mm²/s at room temperature to 1.08 mm²/s and 0.54 mm²/s at 250 ℃ for all the samples used. The generated data set can be used to guide the choice of suitable natural stones to be considered for developing sensible thermal energy storage systems. In addition, this data can be used to analyse variations in thermal characteristics of different types of natural rocks.

Specifications Table

Subject	Engineering & Materials science
Specific subject area	Thermophysical characterisation of solid state, sensible heat storage materials.
Type of data	Table, Graph, Figure Raw, Filtered, Processed
Data collection	Geological map of Zimbabwe was used to locate different natural rocks and nine samples namely, basalt (BA1 and BA2), dolerite (DO1 and DO2), gabbro (GA), granite (GR), rhyolite (RH), gneiss (GN) and quartzite (QU) were used. No pre-treatment was done before experimental measurements of mass and volume of each sample for determining density. Rock samples were crushed into fine powder before loading into the Differential scanning calorimetry, (DSC) apparatus and Thermogravimetric analyser, (TGA) apparatus for measurements of specific heat capacity and thermal stability of the rock samples, respectively. Thermal diffusivity was calculated using experimental data (specific heat capacity and density) and literature data (thermal conductivity)
Data source location	University of South Africa, Florida Campus, Pretoria, South Africa (26° 9′ 2.52″ S, 27° 54′ 9.5″E).
Data accessibility	Data identification number: DOI:10.17632/rxdrhzxfrz.1 Direct URL to data: https://data.mendeley.com/datasets/rxdrhzxfrz/1
Related research article	Seyitini L., Belgasim B., Enweremadu C. C., 2024. Thermophysical characterisation of natural rocks and impact analysis of variations in their thermophysical properties on thermal storage performance. Energy Storage, 6(4):e631. https://doi.org/10.1002/est2.631.

1. Value of the Data

• •Data on thermophysical properties of natural rocks are important when selecting most suitable, cost-effective, solid state sensible thermal energy storage materials which can enable the adoption of renewable heat technologies for industrial applications.
• •Data set on thermal stability of rock samples provides critical information in the determination of suitable temperature range for using rocks safely as heat storage material.
• •Data sets on specific heat capacity and density of rocks are important in the determination of thermal storage capacity and energy density of rock bed-thermal energy storage systems
• •Data set on thermal diffusivity is essential on thermal performance of thermal energy storage systems since it affects rate of thermal charging and discharging of TES systems.
• •Generated datasets can be used by research engineers in thermal engineering sector to analyse performance of solid-state sensible heat storage materials. Also, in the energy sector, these datasets are critical when designing and developing sensible thermal energy storage systems.

2. Background

Determination of this data was motivated by the need to establish thermophysical behaviour of natural rocks available in Zimbabwe. The literature highlighted the potential of natural rocks for cost effective heat storage applications; however, thermal properties of rocks are reportedly influenced by the type and location of the rocks. Therefore, the key objective of generating this data is to determine thermophysical parameters of different rock types and from different sites.

This data set will provide a much-needed build on to the already available literature data to enable comparative analysis of variations in thermophysical characteristics of natural rocks and assess the effects on thermal storage performance of some industrial applications. In addition, the data will enable scientific researchers and engineers to make informed choices when designing sensible or hybrid thermal energy storage systems, using natural rocks as heat storage media

3. Data Description

Thermophysical variables determined for all the considered rock samples are presented in this section and this data is available from the provided direct URL. [1]

(1) Specific heat capacity (determined using raw data)

Data generated on specific heat capacity for the nine samples for varying temperatures is presented in Fig. 1. Data sets with filtered DSC raw data and processed specific heat capacity data for each sample are presented in an Excel file: Combined Filtered DSC raw data and specific heat capacity

Fig. 1.

Variation of specific heat capacity with temperature.

3.1. (2) thermal stability (measured data)

Data on thermal stability determined from the Thermogravimetric analyser (TGA) is presented in graphical form in Fig. 2(a), (b) and (c), while the data sets for all the samples are given in the Excel file: Combined TGA data.

Fig. 2.

Thermogravimetric analysis of (a) basalt samples (BA1, BA2), (b) dolerite (DO1 and DO2), GA, RH and GR samples (c) QU and GN samples.

All samples demonstrated high thermal stability, with a maximum weight loss of <5 % up to 700 °C, confirming their suitability for thermal storage applications. It is noted that the TGA curve for the quartzite (QU) sample (Fig. 2c) shows a slight, anomalous mass gain at temperatures above 400 °C. As this is not accompanied by a corresponding dTG peak, this is attributed to a minor instrumental artifact and does not change the overall conclusion that quartzite is exceptionally stable, with a total mass change of <0.1 %.

(3) Density measurements (measured data)

Experimentally measured data for density of all the studied rock samples is presented in Table 1, while the raw data on mass and volume measurements are provided in the Excel file: Raw data for density measurements.

Table 1.

Experimental data on density of rock samples.

Sample	Average density (kg/m3)	standard deviation ± (kg/m3)
BA1	2995	28
BA2	3019	7
DO1	2836	10
DO2	2857	4
GA	2877	3
GN	2512	10
GR	2736	9
QU	2649	8
RH	2668	5

(4) Thermal diffusivity (calculated from raw and literature data)

Data set on variation of thermal diffusivity with temperature, of all rock samples are presented in Fig. 3 and all the generated data sets are given in an Excel file: Thermal diffusivity data.

Fig. 3.

Thermal diffusivity as a function of temperature.

All covered parameters and their related units are given in Table 2

Table 2.

Variables used and their units.

Variable	Unit
Mass	g
Volume	ml
Density	kg/m3
Temperature	℃
Specific heat capacity	J/kgK
Thermal diffusivity	m2/s
Thermal conductivity	W/mK

4. Experimental Design, Materials and Methods

Nine samples of natural rocks comprising metamorphic and igneous rock types were considered in this data collection process. Rock types used in the study were mapped using the Geological Map of Zimbabwe and samples located mainly in the south-eastern region of Zimbabwe, were considered as shown in Fig. 4, [2]. Description of the samples used is summarised in Table 3.

Fig. 4.

Sites for rock samples used in this study, Adapted from [3].

Table 3.

Description of the characterised rock samples (Adapted from [3].

Sample	Name and location	Type	Description	Sample pictures
BA1	Basalt (30.82 E, −20.10 S)	Igneous	It’s a volcanic type of igneous rocks which has small crystals. It is usually dark coloured because it is made up of dark minerals which include amphibole, biotite, olivine, [4].
BA2	Basalt (31.77 E, −21.24 S)	Igneous	Same description as BA1
DO1	Dolerite (32.63 E, −19.65 S)	Igneous	An intrusive igneous rock. Its texture is fine -medium grained with a similar mineral composition to that of basalt, [5].
DO2	Dolerite (32.40 E, −20.30 S)	Igneous	Same description as DO1
GA	Gabbro (31.16 E, −21.68 S)	Igneous	It is a plutonic rock characterized by large crystals and contains minerals such as, biotite, plagioclase, olivine, and pyroxene, [6].
GN	Gneiss (31.61 S, −20.45 S)	Metamorphic	A coarse-grained rock characterized by parallel bands of mineral grains and foliation. It is formed by metamorphism of other rocks including shale, granite, [5].
GR	Granite (32.23 E, −21.04 S)	Igneous	A plutonic type of igneous rock. It is composed of coarse grains (1 - 4 mm) of quartz and feldspar mixed with dark minerals like biotite, [5].
QU	Quartzite (30.78 E, −19.07 S)	Metamorphic	It is a fine-grained rock and in some types of the grains are too fine to be visible. It is made up mainly of quartz grains and in some cases mixed with mica crystals which are cemented together by SiO2. It is usually found in white colour, [4].
RH	Rhyolite (31.41 E, −21.83 S)	Igneous	It is volcanic igneous rock. It has a similar composition to that of granite rock however it is a fine-grained rock, [5].

A differential scanning calorimetry (DSC) apparatus was used for experimental determination of specific heat capacity of the natural rock samples. The DSC-25 model was used in this experiment and it has a high measurement accuracy with a precision of ± 0.1 %. Rock samples were prepared prior to loading them into the DSC apparatus by crushing them into a very fine powder and an average mass of 3 mg for each sample was used. Each rock sample was thermally charged at a rate of 10°C/minute from room temperature up to 300°C, while nitrogen gas was used for inert conditions at atmospheric pressure.

The DSC apparatus measured and recorded heat flow values as the temperature changed throughout the heating cycle, and this data was used to obtain values for specific heat capacity for each measured sample using Eq. (1), [7,8].

$$c_p=\frac{dQ}{dt}*\left(\frac{1}{m*\beta }\right)$$ (1)

Where $\frac{dQ}{dt}$ is heat flow, m is mass of the sample and β is the rate of heating.

Experimental measurements were done using the thermogravimetric analyser (TGA-550 model,). It is a very accurate method for practical measurements of thermal stability with temperature and weight loss measurements precision of 0.1 % and 0.01 %, respectively. A mean sample mass of 5 mg per rock sample was used in powdered form. Samples were heated at a rate of 10°C/minute from room temperature to 700°C. Nitrogen gas was also used for inert conditions in the TGA chamber. The TGA apparatus monitored and measured the weight losses of the rock samples over the entire temperature range.

The density of the rock samples was determined to represent their 'as received' or bulk wet state. For each of the nine rock types, five independent, irregularly shaped samples were selected. The mass of each sample was measured using an electronic balance with an accuracy of ±0.001 g. The volume of each sample was then determined using the water displacement method with a measuring cylinder capable of resolving ±0.5 ml. The density was calculated as the ratio of mass to volume. The mean and standard deviation for each rock type are reported.

Data on thermal diffusivity (α) was generated from numerical calculations using measured values of density (ρ) and specific heat capacity (c_p) and literature data for thermal conductivity (k), as defined in Eq. (2).

$$\alpha =k/\rho c_p$$ (2)

Limitations

Data set on thermal diffusivity was obtained through calculations using both measured data and data from literature. X-ray diffraction (XRD) was not done to confirm similarity of used rock samples and the rocks covered in literature. The density values presented were measured at ambient temperature. Consequently, the derived thermal diffusivity data, calculated using these values, do not account for changes in density due to thermal expansion or mass loss at elevated temperatures. This should be considered a source of uncertainty when applying the data to high-temperature models.

It is important to note that the thermal diffusivity values presented in this dataset were not measured directly. They were calculated using experimentally determined values for density and specific heat capacity, but with thermal conductivity values sourced from literature. As thermal conductivity is highly dependent on the specific mineralogy and microstructure of a rock, and since no mineralogical analysis (e.g., XRD) was performed on our samples, the use of literature data introduces a significant source of uncertainty. Therefore, the thermal diffusivity data should be interpreted as a first-order estimation, useful for comparative analysis between the samples studied, rather than as an absolute measurement.

Ethics Statement

In this study, authors did not conduct human or animal studies.

Acknowledgements

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Luckywell Seyitini: Writing – original draft, Methodology, Investigation, Formal analysis, Conceptualization. Basim Belgasim: Writing – review & editing, Supervision, Methodology. Christopher Chintua Enweremadu: Writing – review & editing, Supervision, Resources, Methodology, Conceptualization.

Data Availability

• Mendeley DataData sets on thermophysical characteristics of natural rocks for thermal energy storage (Original data)