Salt's Thermophysical Parameters and Thermal Energy Storage Costs

Salt's Thermophysical Parameters and Thermal Energy Storage Costs: History

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Subjects: Chemistry, Inorganic & Nuclear

Contributor:

Marta Styś-Maniara

Magnesium nitrate hexahydrate (MNH) has the appropriate phase transition temperature for use in solar thermal energy storage. Understanding the thermophysical parameters of the substances, supported by the results of experimental studies in solar installations, may prove helpful for the environmental assessment of these substances.

inorganic salt hydrates
environmental
public health

1. Introduction

Since the advent of renewable energy sources, they have been gaining greater and greater acceptance by society, especially when it comes to solar energy [1]. Local supply of this type of energy, with the use of distributed energy systems, plays an important role in the global infrastructure [2]. In some regions of the world, the potential environmental impact of different technologies is still little known [1]. On the one hand, replacing conventional sources (coal) with renewable energy has a positive effect on climate change and reduces soil acidification and eutrophication, but, on the other hand, technological advances and the introduction of new substances in installations, the impact of which is unknown in the long term, may cause serious environmental risk or social dissatisfaction [2]. The knowledge of the environmental effects of the interactions of various substances already at the stage of their selection for use in a photovoltaic installation in connection with their technical, economic, and social properties should be an important aspect in the process of designing such solutions [3,4].

Photovoltaics is the fastest growing field of energy generation. Photovoltaic panels absorb 80% of incident solar radiation, 20% of which is converted into electricity, and the rest is converted into heat, causing an increase in the temperature of the panel surface. The efficiency depends on several factors, such as: the material from which the panel is made, the intensity of the solar radiation, the surface temperature, the angle of the panel, and the wind speed. For each degree of temperature increase, the efficiency of the PV decreases by 0.4–0.5%. The panels operate most efficiently when they reach a nominal temperature close to 25 °C [4,5,6]. Scientists show that using the right phase change materials (PCM) will improve the performance of the photovoltaic (PV) panels. The storage of thermal energy in a photovoltaic system consists in the exchange in heat as a result of a phase change taking place in a suitable material and at an appropriate temperature. The thermal conductivity of the phase change materials is very low and ranges from 0.16 to 0.25 W/mK. Such an action lowers the surface temperature of the PV and increases the efficiency of the system [7].

One of the groups of phase change materials that are used in solar installations includes inorganic salts [8]. Inorganic salts, as opposed to organic ones, are considered more environmentally friendly and safe [9], have a higher volumetric latent heat, are cheaper, more easily available, their thermal conductivity is higher, they have lower volumetric changes, and they are a non-flammable material. However, the disadvantage is that they are corrosive, they are subject to overcooling, and the volume changes are high [8]. Taking care of the technical and economic aspects, there are works devoted to improving the properties of salt hydrates. One of the most promising trends is nanoencapsulation, which improves their structural properties and reduces costs and increases the potential of their applications [4,9,10]. Research is still being undertaken to create mixtures of salt hydrates that will lower their corrosiveness, as well as reduce or stabilize supercooling and volume changes [9,10,11,12]. Researchers are working on improving the environmental properties of PCMs. Elias and Stathopoulos, in their publication [4], wrote that energy efficiency is of key importance for maintaining the competitiveness and profitability of investments in solar systems. This, in turn, should take into account current or future environmental regulations. Despite the extensive literature on salt hydrates used as PCM materials in solar installations, little is known about the potential environmental effects of their use. Researchers usually only indicate that the environmental aspect is important [13,14,15,16], but there are no detailed studies. The present authors did not find any work that would analyze the technical, economic, environmental, and social aspects in the context of the use of non-organic salt hydrates for applications in solar installations. However, only full knowledge based on all these factors will allow a rational selection of appropriate salt hydrates and their mixtures and will enable sustainable development.

2. Thermophysical Parameters of the Salt and the Costs of Thermal Energy Storage

In phase change materials, such as salt hydrates, latent heat is stored, consisting of a phase change in the heat-storing material. During the change in the state of aggregation, a large amount of energy is accumulated, and the amount of heat exchanged is described by Formula (1).

Q = m[c_ps(T_m − T_i) + h_f + c_pl(T_f − T_m)] (1)

where:

m—mass of the substance [kg],

c_ps—specific heat of the solid [kJ/(kg∙K)],

T_m—phase change temperature [K],

T_i—initial temperature of the heat storage process [K],

h_f—specific enthalpy of phase transition [kJ/kg],

c_pl—specific heat of the liquid [kJ/(kg K)],

T_f—final temperature of the heat storage process [K].

Most experiments with salt hydrates in photovoltaic systems focus on the latent heat absorption of the phase transition, i.e., the thermal energy absorption capacity for a solid-to-liquid phase transition of a substance. This is beneficial in temperature control, as salt hydrates assist in the passive cooling in photovoltaic modules [9].

Understanding the thermophysical parameters of the substances, supported by the results of experimental studies in solar installations, may prove helpful for the environmental assessment of these substances, which is described in the following sections. When analyzing the thermophysical parameters, it is advantageous if the substance accumulates large amounts of energy at a constant temperature or in a limited temperature range corresponding to the phase change in a given material. Moreover, the PCM phase change temperature should be within the temperature range of the given application. In turn, higher values of energy storage density translate into a smaller volume of material necessary to accumulate a given amount of heat, which may minimize the amount of waste [17]. The thermophysical parameters of the analyzed salts are presented below, based on the information contained in the safety data sheets and on the basis of experimental studies by various authors (Table 1).

Table 1. Thermophysical parameters of salt hydrates for use in solar installations and their cost.

CAS Number	Salt Hydrates	Melting Temperature °C	Heat of Fusion J/g	Density g/cm³	Cost €/kg
7791-18-6	MgCl₂·6H₂O	116.7 [18] 115 [19]	166.9 [19]	1.57 [18]	164.9 [20]
Magnesium Chloride Hexahydrate		116.7 [18] 115 [19]	166.9 [19]	1.57 [18]	164.9 [20]
13446-18-9	Mg(NO₃)₂·6H₂O	89 [15,21]	162 [15]	1.64 [21]	115.6 [20]
Magnesium Nitrate Hexahydrate		89 [15,21]	162 [15]	1.64 [21]	115.6 [20]
7727-73-3	Na₂SO₄·10H₂O	32.4 [10,22]	251 [10]	1.46 [22]	104.7 [20]
Sodium Sulfate Decahydrate		32.4 [10,22]	251 [10]	1.46 [22]	104.7 [20]
6131-90-4	CH3COONa·3H₂O	57.9 [23] 58 [24]	260 [25] 250 [26]	1.45 [23]	112.6 [20]
Sodium Acetate Trihydrate		57.9 [23] 58 [24]	260 [25] 250 [26]	1.45 [23]	112.6 [20]
6132-02-1	Na₂CO₃·10H₂O	30 [27] 32.4 [8]	247 [8]	1.44 [27]	47.4 [20]
Sodium Carbonate Decahydrate		30 [27] 32.4 [8]	247 [8]	1.44 [27]	47.4 [20]
10039-32-4	Na₂HPO₄·12H₂O	35 [28,29]	256.6 [28] 278.8 [29]	1.52 [28,30]	40.2 [20]
Disodium Hydrogen Phosphate Dodecahydrate		35 [28,29]	256.6 [28] 278.8 [29]	1.52 [28,30]	40.2 [20]
12230-71-6	Ba(OH)₂·8H₂O	78 [31,32]	233–332 [32]	2.18 [31]	127.1 [20]
Barium Hydroxide Octahydrate		78 [31,32]	233–332 [32]	2.18 [31]	127.1 [20]
7774-34-7	CaCl₂·6H₂O	30 [33] 28.6 [34] 29 [35]	169.5 [34] 160 [35]	1.71 [33] 1.83 [35]	31.4 [20]
Calcium Chloride Hexahydrate		30 [33] 28.6 [34] 29 [35]	169.5 [34] 160 [35]	1.71 [33] 1.83 [35]	31.4 [20]

Höhlein et al. [19] consider magnesium chloride hexahydrate a promising material for practical applications due to its good thermophysical properties. According to the authors of [19], the substance will also prove useful in waste heat transport systems. In turn, according to Saikrishnan et al. [36], the discussed hydrate is not the best for solar systems due to its low thermal conductivity. The authors analyzed the thermal performance of a solar-powered thermal energy storage (TES) system with MgCl₂·6H₂O. The material was sealed in copper cylindrical containers and placed vertically in the TES reservoir. The ability to store energy was assessed. Low thermal reactions of the components and low temperature changes in the morning were observed. The maximum efficiency of the solar collector was 72.5% between 1:00 p.m. and 2:00 p.m. and it decreased with the decrease in solar radiation.

Magnesium nitrate hexahydrate (MNH) has the appropriate phase transition temperature for use in solar thermal energy storage [37]. The thermophysical properties of MNH improve with an increase in the mass percentage of the carbon sphere as a filler. The optimal value is 0.5% by mass of the carbon material, and it improves the thermal conductivity of the MNH and the heat transfer coefficient. The authors obtained the best parameters for the composites with a carbon sphere and worse for nanographite [37]. The energy storage density is significantly improved when the eutectic substance, magnesium chloride hexahydrate, is used together with magnesium nitrate hexahydrate. However, low thermal conductivity limits the rate of heat uptake [38].

Li et al. [39] and Shein et al. [40] consider sodium sulfate decahydrate to be a suitable candidate for heat storage applications due to its high latent heat and desirable phase transition temperature, but it suffers from undercooling and phase separation. Similar limitations also exist with sodium acetate trihydrate (SAT) [9,24,41]. However, in the case of SAT, the high degree of supercooling and the high energy storage density make it an ideal flexible material for storing heat with almost no heat loss in both the short and long term, which provides enormous benefits for the energy system [42].

Rathold et al. [43] showed in their research that sodium carbonate decahydrate and calcium chloride hexahydrate are useful in thermal energy storage systems, but sodium carbonate decahydrate has a better efficiency of approximately 5–7% during charging and discharging. Pichandi et al. [44] created a eutectic mixture of sodium carbonate decahydrate with magnesium sulfate heptahydrate to improve the thermophysical properties of the PCM, including the latent heat of fusion and the phase transition temperature. In the PV-PCM system, an increase in the daily DC output power of 12.5% was observed as compared to the reference PV module.

Disodium hydrogen phosphate dodecahydrate (DHPD) is considered an excellent hydrated salt for storing latent heat due to its high latent heat and almost no phase separation. Its greatest disadvantages are supercooling and dehydration after exposure to air [28,45]. The leakage problem can be prevented by encapsulating it in porous expanded vermiculite (EV) [28]. DHPD is used in long-term solar heat storage systems where a higher storage efficiency is generated than in sensible heat storage systems. The efficiency of the system improves with the extension of the heat storage period [46].

Barium hydroxide octahydrate (BHO) is a salt with a high heat capacity but exhibiting a significant phenomenon of supercooling and phase separation during the cooling process, which limits its use. These problems can be minimized thanks to the use of nucleating and gelling agents [31,47]. Han et al. [48] increased the thermal conductivity of BHO by two to four times, creating composites with expanded graphite (EG), with little negative influence on other properties. The authors consider the material ideal for a solar energy storage system within finite thermal cycles.

Calcium chloride hexahydrate is compared by Pan et al. [49] to paraffinic PCM due to a high latent heat, thermal conductivity, and the possibility of storing excess heat. The substance melts at 29.6 °C, which can ensure the material melts when it absorbs excess heat from solar radiation. A serious problem is supercooling, which is minimized by means of nucleating agents, e.g., BaI₂·6H₂O, SrCl₂·6H₂O and SrBr₂·6H₂O [50]. In contrast, Donkres et al. [13] believe that calcium chloride hexahydrate does not meet the requirements for seasonal heat storage (domestic heating, domestic hot water). According to the authors [13], it is impossible to achieve the required temperatures during hydration with a reasonable energy storage density.

The research shows that the proposed salt hydrates are good candidates for applications as a PCM material in solar installations in terms of the thermophysical parameters. However, several authors point out the particular usefulness of SAT [42], BHO [48], and DHPD [28]. However, it should be borne in mind that each salt hydrate, apart from its advantages, has its limitations. Choosing the right substance is often a compromise. Methods are also sought to improve the thermophysical properties of the salt hydrates as outlined above. This is also confirmed by the authors of [10,51], who write that currently there is no material available that would meet all the requirements for commercial applications. Moreover, Song et. al. [10] indicate the limitations we must take into account when selecting a specific group of salt hydrates. According to the authors, nitrates have a low corrosivity, but also a low thermal conductivity, which can easily overheat. Carbonates have a low corrosivity and a high density and solubility, as well as a high melting point, but some break down easily and have a high viscosity. Chlorides, on the other hand, are corrosive.

Various authors have written about the relatively low cost of salt hydrates [25,30,42,46]. However, the prices of inorganic salt hydrates are compared to other PCM compounds and not to each other. It seems important to know the differences in the case of inorganic salt hydrates for the same type of application (solar installations), which may be important when choosing a given substance. The prices of the salt hydrates were determined on the basis of the price list available on the Sigma Aldrich website as of 01/07/2022 [35]. The most expensive salt hydrates are magnesium chloride hexahydrate, barium hydroxide octahydrate, magnesium nitrate hexahydrate, and sodium sulphate decahydrate. The cheapest ones are calcium chloride hexahydrate, disodium hydrogen phosphate dodecahydrate, and sodium carbonate decahydrate. The higher the density of a given salt hydrate, the smaller the volume of waste that can be generated in the future.

The most advantageous from an economic point of view, due to the relation of the price and density, are calcium chloride hexahydrate, in the first place, and disodium hydrogen phosphate dodecahydrate, in the second place. On the other hand, magnesium chloride hexahydrate and sodium acetate trihydrate seem to be the most unfavourable and can generate the largest volume of waste.

In order to ensure the conditions of the safe use of the equipment at high temperatures and under high pressure in terms of thermal energy storage, salt hydrates should meet a number of requirements. From the point of view of the environment, it is important to assess the chemical stability, toxicity, and flammability in the temperature range used [10]. Therefore, in the following chapters, the authors will discuss the environmental properties of the salt in two ways: first, by analyzing the environmental and health problems caused by chemical hazards on the basis of the available material safety data sheets. Secondly, by analyzing the potential disadvantages of the salt hydrates in terms of the environmental hazards based on the results of experimental studies available in the literature.

This entry is adapted from the peer-reviewed paper 10.3390/ijerph20021331

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