2 Storage options for energy and categorization of PCMs
Latent heat, thermochemical energy, and sensible heat, or a mix of these, can be utilized to keep thermal energy as a shift in a material’s intrinsic energy. With only a little variation in temperature, PCM has the ability to retain a significant amount of heat in a tiny volume and mass
Fig. 2 TES categorization [32] .
2.1 Storage options for energy
There are numerous chemical or physical methods for storing thermal energy, and each one has unique properties. The heat that is retained as a result of a substance’s temperature shifting is known as sensible heat. Heat capacity is defined as the relationship between the heat that has been stored and the temperature differential. Utilizing a sensor, such as a thermometer, is the simplest approach to signal and measure. Latent heat or the heat a phase transition absorbs or releases is another way that heat can be retained.
2.1.1 Sensible heat storage (SHS)
By increasing the temperature of a solid, liquid, or gas, thermal energy can be stored through sensible heat storage (SHS). When charging/discharging, SHS units make use of the material’s heat capacity and internal temperature. Depending on the medium’s specific heat, the variations in temperature, and the quantity of storage substance, different amounts of heat can be stored
(1)
2.1.2 Latent heat storage (LHS)
LHS is on the basis of the emission/absorption of heat that occurs as a storage substance transitions between solid and liquid, or between liquid and gas. LHS is a fascinating subject because it offers high-density energy storage and the ability to store energy throughout a narrow temperature range or at a fixed temperature; this is the degree of temperature at which the material experiences its phase transition, which corresponds to the material’s phase transition temperature Using a PCM medium, the LHS unit’s storage capacity is calculated as follows:
(2)
Phase changes can occur in three different ways
This phase transition occurs via an isothermal mechanism, and often, among the two phases, there is a slight volume change. To switch between solid and liquid phases, the distinction between the heat variation of latent and sensible is shown in Fig. 1 with clarity. The temperature doesn’t change throughout the phase transition and heat is kept in large quantities. It is among the most significant properties because the melting values of PCM materials vary. Any additional heat source will only result in an increase in perceptible heat if the PCM is entirely melted. The application and required temperature range are taken into consideration when choosing PCM.
Only under extreme heat supply conditions does this occurrence take place.
Being able to store a significant amount of heat in a vast number of molecules only occurs in a few substances in nature, making it highly unusual. However, the LHS is typically lower. It behaves like solid-to-liquid conversions.
Comparatively speaking, the latent heat of converting solid to liquid is lower than that of liquid-gas or solid-gas. But just a little volume change (of about 20% or lower) is involved in these changes. Transitions between solid and liquid have been shown to be economically desirable for application in thermal energy storage units PCMs cannot serve as a heat transmission medium by themselves. A heat exchanger must be utilized in between and a different heat transfer medium for the purpose of transferring heat between the PCM and the heat sink, and between the PCM and the heat source. Generally speaking, PCMs have low heat conductivity, and the heat exchanger that will be employed must be properly developed. Consequently, the following three elements must be included in an LHS unit: An appropriate PCM whose melting point is within the necessary temperature range, a PCM-compatible heat exchanger, and container.
2.2 Categorization of PCMs
Investigating and classifying the traits is crucial to maximizing PCM’s potential. In nature, phase changes occur in the majority of materials. As an example, over the limit of boiling, water evaporates from the liquid state and becomes ice below the freezing point. While all of these materials have the potential to be PCMs, only a select few can actually do so as boosters of thermal efficiency. The efficient development of the ideal PCM depends on PCM classification, which is useful for systematically determining characteristics. According to their constitution, the majority of PCMs fall into one of three types—either eutectic, organic, or inorganic—as depicted in Fig. The classification reveals clear traits as a result of the various molecular configurations. As a result, this classification technique is frequently employed. The majority of organic PCMs are comprised of paraffin and esters; fatty acids, alcohols, and glycols are examples of additional nonparaffin compounds. Metallics and salt hydrates make up the group of inorganic PCMs; additionally, eutectics may contain combinations of PCMs that are both inorganic and organic, as well as inorganic and inorganic
Fig. 3 Categorization of PCMs used for LHS
2.2.1 Organic PCM
Because they have a variety of chemical configurations, most of which are hydrocarbon compounds, organic PCMs exhibit various thermal characteristics. According to Fig. the subcategorization of organic PCMs has been changed to “alkane” as well as “others” if the paraffin is assumed as a specific type in comparison to the categories of “paraffin and nonparaffin” Paraffin, which is made up of a combination of alkanes containing 20–40 carbon atoms, is the most prevalent organic PCM and has received a lot of attention The term “other” as a type of organic PCM, on the other hand, mostly refers to a pure material, which is why paraffin has been reclassified as a specific kind as depicted in Fig. For thermal storage, organic PCMs are frequently utilized as a result of they have a variety of beneficial characteristics, such as reutilizability noncorrosiveness, minimum upfront costs, elevated latent heat and little or no supercooling Fatty acids and paraffin have been under extensive study in the literature among organic substances.
Fig. 4 Organic PCM classification
At ambient temperature, the pure alkanes exhibit a clear phase transition phenomenon that enables them to function as PCMs. Some of the substances have a general chemical equation in general similar to n-eicosane cetane hexadecane heptadecane and have evidence to support their effectiveness as PCMs. Typically, alkane melting points grow as the carbon atom count or relative mass of molecules increases. The chemical equation for hexadecane [73] is and its phase transition point is between 18 and 20 degrees Celsius with a 224kJ/kg enthalpy, a family member of the alkanes known as a saturated hydrocarbon that has good compatibility and doesn’t react chemically with other substances. As the relative mass of molecules rises, n-eicosane [71] and heptadecane [74] had respective melting points of 37.27°C and 22.91°C; however, n-eicosane has a lower enthalpy. Additionally, the integration operability of these components was good, and there was no chemical reaction with the supporting substances. One of the best substances for storing latent heat is paraffin, especially in solar thermal usage. The most amazing features that lead to its widespread use are a limited range of temperatures and significant latent heat, nontoxicity, wide availability, and reasonable pricing. The characteristics of the paraffin can be maintained over 1000–2000 heat cycles A specific kind of CnH2n+2 is paraffin, i.e., hydrocarbons with straight chains and saturation that melt between 23°C and 67°C. Due to its lower TC (0.15–0.28W/mK paraffin presents the most challenge in use. Because paraffin is a blend of several alkanes, it demonstrated a range of characteristics in tests According to Meng et al. the PCM made from paraffin demonstrated the benefits of using an appropriate temperature and enthalpy of phase transition, dependability, optimal performance in temperature regulation, and strong thermal stability. For the appropriate phase transition enthalpy and temperature, the melting points of paraffin varied and may be changed to a desired value. In order to heat flow prevention in the summertime, Qu et al. [67] chose paraffin that melts at about 44°C has a changeable range of 35–44°C and has a 260kJ/kg useful latent. Widespread evidence supports strong thermal stability. To create a unique PCM composite, Ramakrishnan et al. [80] paraffin RT21, a product of the Rubitherm firm, which is made of saturated hydrocarbons. Following 100 heat cycles between 15°C and 35°C, in the summertime in Australia, temperature ranges frequently vary, and despite a small drop, the paraffin RT21 nevertheless exhibited a steady energy storage capacity. It was widely accepted for its efficiency in temperature regulation For illustration, P56-58 paraffin wax manufactured by Merck was utilized by Khan et al. Between a warm and cool bath of water, the PCM might have lowered the heat flux by over 17.2%, resulting in less variation in temperature and a lengthy delay of over 40min to achieve the target temperature. Using paraffin to heat air with solar energy has been the subject of numerous investigations that have been documented in the literature. The effects of linking the SAHS and PCM were experimentally studied by Kabeel et al. As a solar collector, a SAHS with a rectangular channel was employed and PCM was crammed against the channel’s lower wall. When compared to the conventional system, it was discovered that the system’s effective usage time had increased by 4h. With an absorber surface with V-corrugation Kabeel et al. [86] discovered that the unit thermal efficiency enhanced by 15.3% to 21.3% in a different investigation on the same unit. Shalaby et al. [87] altered a similar unit by adding a top cover to limit heat losses at night and achieved the desired results. Besides organic PCMs, with the exception of mixed and all-pure alkane, both organic alcohol [88] and organic acid make up the majority. Fatty acids are important organic acids, which have long aliphatic chains of carboxylic acid, including both saturated (lauric, capric, caprylic, etc.) and unsaturated (sapienic, palmitoleic, myristoleic, etc.) fatty acids. In order to analyze SAHS with LHS, Wadhawan et al. [91] performed both numerical and experimental studies. Copper tubes that were positioned inside the air duct that is rectangular were filled with lauric acid, which was utilized as PCM. Lauric acid was found to greatly improve thermal effectiveness and duration under effective usage. Mehla and Yadav [45] conducted an experimental examination of an SAHS that included thermal storage and an ETSC to determine how well it performed thermally. The thermal storage employed acetamide as PCM. The system was determined to have very excellent performance and it was discovered that the output temperature was 20.2°C and 37°C above the surrounding temperature, throughout hours of darkness (without sunlight) and daylight, respectively. In a different study, Ait et al. [92] used an experimental evaluation to compare the effectiveness of two distinct PCMs to heat space with solar energy: n-octadecane and capric acid. Thermal storage was accomplished using spherically PCMs-packed beds that are enclosed. It was discovered that by using the developed technology, wintertime electricity usage may be cut by 32%.
2.2.2 Inorganic PCM
Metallics or salt hydrates make up inorganic PCM. Strengthening the melting point, greater LHS, inflammability, and strong heat conductivity of inorganic PCMs are just a few of their many favorable qualities Inorganic PCMs have a few drawbacks, including being corrosive, substantial volume shift during phase transition, and supercooling Due to the salt hydrates having an ideal melting point, they have been used more frequently than metallics among inorganic substances for applications utilizing solar thermal. Chemically inorganic substances called salt hydrates are always accompanied by molecules of water integrally. The related molecules of water are typically referred to as “water of hydration.” Notable capacity for latent heat and strong TC are favorable PCM characteristics that salt hydrates have In order to better understand the crucial characteristics of PCM made of sodium nitrate Bauer et al. [93] researched and examined the experiment outcome. In addition to extending the temperature range for nitrite formation in molten the data on its thermal and physical properties showed that it was a reliable PCM. Han et al. [94] studied a system that used solar energy and to store and pump heat. Results demonstrated that adding thermal storage to the unit increased its COP.
2.2.3 Eutectic PCM
A eutectic PCM with certain characteristics can be created by combining at least two PCMs. Eutectics are at least two compounds that have the lowest boiling mixtures and exhibit steady freezing and melting, and the term “eutectic composition” refers to the composition that corresponds to them. Their primary benefit is the extremely low-temperature range for phase changes There are several organic/organic combinations that are eutectics and demonstrate different appealing characteristics. Organic eutectics, in particular, have been extensively researched for use as PCM in air cooling usages; however, there are relatively few research works available for its use in air heating powered by solar energy. As an and ZnO-coated eutectic PCM, Kawaguchi et al. [96] used Al alloy with 30wt% Zn. At temperatures above 400°C, it demonstrated a strong ability as a substance for storing heat; nonetheless, it is unsuitable for use in construction materials. Eutectic PCMs that are inorganic/inorganic and other eutectic alloys had the same issue containing molten salt that is a ternary hybrid At a working temperature greater than the melting point, according to various morphological types and integration techniques, Miliozzi et al. [97] studied the characteristics of solar salts used in cement mortars serving as PCM and comprised of 60% and 40% On the other hand, eutectic PCMs that are organic/organic have been successfully incorporated into building components and have shown significant improvements. Meng and Wang [79] developed a PCM with eutectic form stability, consisting of a fatty acid mixture of lauric and capric chains (66:34 mass fraction for CA-LA). The rest of the eutectic PCMs are made up of both inorganic and organic PCMs, and they may be able to bridge the gap between them by having the right characteristics. In order to create an STP-SAT hydrated salt eutectic PCM, Chen et al. [99] introduced sodium thiosulfate pentahydrate, or STP, and sodium acetate trihydrate, or SAT. With a 72:28 mass ratio, STP and SAT have favorable temperatures of phase transitions and high enthalpies.
