**1. Introduction**

Intensive combustion processes of conventional petroleum-based fuels pose a significant impact on the environment in the long term and in the vicinity of residential areas due to exposure to harmful concentrations of gaseous emissions, namely COx, SOx, and NOx. As a result, stringent environmental regulations against these gaseous emissions and the operability of thermal combustion facilities to reduce environmental impacts are legitimized [1]. Due to the rising costs of petroleum end-products and increased demands in thermal applications by the residential and industrial sectors, researchers have been encouraged to investigate new resources of renewable energies (REs) and develop ecoenvironmentally friendly REs such as photovoltaics (PVs) and thermal solar panels (SPs). The installation of solar panels on wide terrain is intended to collect solar energy during the day. Moreover, the current thermal energy demands strongly encourage researchers to explore promising engineering solutions for effective thermal energy depots and dispatching solutions. Thermal energy storage has become increasingly crucial, owing to its interaction with variable production resources, the increase in the demand for conventional fuels for the combustion process, and the adverse environmental impact of other RE sources. Therefore, the ideal way to balance thermal energy is for it to be stored in conservative depots utilizing phase change materials such as paraffin based PCMs, which are ecologically and economically ideal.

**Citation:** Malkawi, D.S.; Rabady, R.I.; Malkawi, M.S.; Al Rabadi, S.J. Application of Paraffin-Based Phase Change Materials for the Amelioration of Thermal Energy Storage in Hydronic Systems. *Energies* **2023**, *16*, 126. https:// doi.org/10.3390/en16010126

Academic Editor: Gianpiero Colangelo

Received: 4 December 2022 Revised: 16 December 2022 Accepted: 19 December 2022 Published: 22 December 2022

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Thermal energy storage is a feasible compensation for fluctuations between production and consumption rates during peak demand periods through thermal energy depot facilities that could be integrated within RE producers' and consumers' buildups. The integration of PCMs with an energy storage system has several potential applications, including the intensive and cumulative latent heat of phase changes. Furthermore, the phase change process is compatible and better monitored, since it occurs ideally at isothermal temperatures [2]. Despite these REs' potential, they possess a few deficiencies, such as crisp efficiency and less availability than other RE sources such as wind, traditional solar, and substrates for biofuel production [3]. The availability of sunlight varies across continents and between the earth's upper and lower hemispheres, potentially influencing energy availability.

The PCM products can be classified into three categories: eutectic, organic, and inorganic materials [4–9]. Organic PCMs include paraffin and non-paraffin. The main advantages of organic materials are changing their phase without segregation and latent heat degradation; self-nucleation; non-corrosiveness; chemical stability and safety. Inorganic PCMs include salts, hydrates, and metallic materials. They have a high storage density, high thermal conductivity, are non-flammable, and are readily available, but they need a nucleation agent and have a super-cooling problem in the phase transition. Eutectics are mixtures of two or more components [4–9].

Hydronic systems are usually associated with liquid water as a heat transfer medium for the cooling and heating processes. A hydronic system typically includes both cooled and heated water cycles to allow for separate heat transfer. Typical temperature differences of such systems are within the range of 0 and 15 ◦C for cooling and between 20 and 100 ◦C for heating [10–12]. Recently, solar water collectors have been considered a significant alternative to traditional electric heaters in meeting domestic hot water requirements. Although solar water heaters are composed of various types, passive or natural convection types are used widely due to their simplicity and operational efficiency [13,14].

The development of traditional solar heating and cooling systems was reviewed in Ge et al., 2018; storing excess heat for further applications was recommended, and enhancements to the solar energy storage system were highlighted [15]. Moreover, Buker et al., 2015 discussed improvements in solar panel design, such as panel surface, tilt, and shading, that could have a significant influence on the performance of the integrated hydronic systems [16]. Nevertheless, the obstacle that limits the solar water collectors is the scarcity of matching demand and supply throughout the day. The operation of solar water collectors depends on the availability of the sun [17] and heat losses [18].

Several researchers have confirmed that thermal energy storage is an essential issue by using appropriate thermal storage material within the solar energy system, which could be incorporated in a storage tank [19–21] or with collector tubes [22,23]. Recently, the heat that is absorbed or released during a phase change of PCMs has been employed as a thermal storage battery, due to its higher latent heat, wide operating temperatures, and very good thermal properties [24–31]. A PCM absorbs and stores thermal energy during the sunny hours of the day; later, it releases the stored energy after the sun's absence, which improves the solar system's efficiency. Organic PCMs, such as paraffin wax, are best known for storing a large amount of energy due to their high latent heat, thermal and chemical durability, little sub-cooling, and non-toxicity [32,33]. In the recent literature, the thermal behavior of paraffin-based PCMs was studied for the energy depot process. Murali et al., 2015 have examined the effectiveness of flat-plate solar water collectors incorporating paraffin as a PCM in a container placed in the top section of the water tank. Their findings appear to improve the performance of the solar system [34]. Kumar et al., 2020 have investigated the behavior and effect of applying synthesized nano-PCMs on the energy storage of evacuated solar water heating systems. According to their findings, PCMs were filled in evacuated solar tubes, which were connected to cylindrical containers placed inside the water tank [21], and such PCMs flowed as liquid inside and served as an energy storage medium to heat the water inside the main tank. In

a previous study, we investigated the thermo-physical properties of PCMs by studying the enhancement of the thermal conductivity of the heat transfer medium of a PCM with the addition of carbon nanotubes (CNT) and graphite nanoparticles (GNP) as nanofillers to PCM composites [35]. So, future outcomes will focus on the enhancement of the performance efficiency of the solar system by adding nanoparticles to the PCM, which are then incorporated into the system. In this context, prior studies [36–38] have addressed the application of a shell and tube thermal storage heat exchanger equipped with finned outer walls for the tubes, and the enthalpy-porosity method was utilized to reveal the transient behavior of the PCMs' melting process. This approach could be subject to various complexities, and several criteria must be met to apply the proposed enthalpy-porosity method. In addition, the wavy annulus tubes could cause apparent vortices inside the heat exchanger that affect the natural convection of heat transfer.

Generally, the reviewed studies imply that the integration of PCMs within a solar system could ameliorate the performance of the thermal mass, maximize operational simplicity, and recover the thermal energy of the hydronic solar system for off-peak periods. It could be understood that few attempts were made to establish an in-field hydronic system that has a potential application of heating water in residential and industrial premises and to replace conventional electrical/fuel-based water heating systems, thereby improving energy storage efficiency during off-peak periods and reducing relevant energy expenses in premises. This study may offer guidance for future research and the thermal design of domestic hydronic solar systems. The performance of the system is assessed with an integrated PCM that is distributed on the shell side of the water storage tank, such that the PCM shell has a different thickness at the top and bottom of the storage tank (the bottom portion is thicker than the top). The effect of the PCM in a natural circulation solar water collector was examined through normal domestic hot water consumption, complete and sudden emptying of the hot water storage tank, and no hot water consumption.
