A Review of Phase-Change Material-Based Thermal Batteries for Sustainable Energy Storage of Solar Photovoltaic Systems Coupled to Heat Pumps in the Building Sector

Abstract

Buildings account for about a third of global energy and it is thus imperative to eliminate the use of fossil fuels to power and provide for their thermal needs. Solar photovoltaic (PV) technology can provide power and with electrification, heating/cooling, but there is often a load mismatch with the intermittent solar supply. Electric batteries can overcome this challenge at high solar penetration rates but are still capital-intensive. A promising solution is thermal energy storage (TES), which has a low cost per unit of energy. This review provides an in-depth analysis of TES but specifically focuses on phase change material (PCM)-based TES, and its significance in the building sector. The classification, characterization, properties, applications, challenges, and modeling of PCM-TES are detailed. Finally, the potential for integrating TES with PV and heat pump (HP) technologies to decarbonize the residential sector is detailed. Although many studies show proof of carbon reduction for the individual and coupled systems, the integration of PV+HP+PCM-TES systems as a whole unit has not been developed to achieve carbon neutrality and facilitate net zero emission goals. Overall, there is still a lack of available literature and experimental datasets for these complex systems which are needed to develop models for global implementation as well as studies to quantify their economic and environmental performance.

1. Introduction

Energy plays a vital role in economic growth and development, driving the expansion of industry, public services, and transportation [1]. The need for energy is growing as the world’s population and living standards rise [2]. According to the International Energy Agency, the world’s total energy consumption has doubled over the past 50 years [3]. Even though the rate of development has decreased compared to prior years, energy demand is expected to rise by 30% between today and 2040, equivalent to adding the present energy demands of China and India combined [4]. The building sector, as a major energy consumer, accounts for nearly one-third of the global final energy consumption and half of all electricity usage, contributing significantly to both direct and indirect carbon dioxide (CO₂) emissions associated with energy use [5]. In residential buildings, approximately 70% of energy consumption is dedicated to space heating and appliances in colder climates, while in moderate and warmer climates, it is used mainly for water heating and cooking [5].

Globally energy generation is dependent on fossil fuels, which account for around three-quarters of global greenhouse gas (GHG) emissions, leading to climate change worldwide [6]. In total, 92% of the CO₂ emission comes from burning fossil fuels, which contain 74% of GHG [7]. In the energy sector, electricity and heat are responsible for emitting 29.7%, whereas buildings account for 6.6% of GHG [7]. When it comes to residential sectors, they have the largest share of energy sector emissions with a global 12.5% [7]. GHG emissions are also responsible for at least five million deaths caused by air pollution every year [6].

For sustainable and clean energy solutions, it is imperative to switch to renewable energy sources due to the limited availability of fossil fuel reserves, their rising costs, and the greenhouse gas emissions they produce upon combustion [8]. This transition will help achieve the carbon neutrality goal, which states a balance between the emission and absorption of carbon from the atmosphere [9]. Among renewable energy resources, solar photovoltaic (PV) technology has incredible promise as an environmentally friendly resource because it does not harm the environment when in use [10], it is a well-known route to a sustainable state [11], it allows the generation of solar electricity [12], it has no moving parts [13] and it has low maintenance costs [14]. It can replace fossil fuels in building and residential sectors, helping reduce carbon footprint and contribute to carbon neutrality, as residential solar panels emit only 41 g of CO₂ equivalent emissions per kWh of energy generated [15]. Due to the ongoing decline in the price of PV systems, solar energy has recently emerged as the most affordable sustainable form of electricity generation [16] and it is anticipated that PV costs will keep falling [17]. Thus, it is not surprising that PV is the fastest-growing energy source globally [18].

The important characteristics of solar PV systems, i.e., self-consumption and self-sufficiency, can be improved by the integration of heat pumps (HPs) and energy storage [19,20]. On the other hand, the intermittent nature of renewable energy in general and solar in particular makes it difficult to meet the strict and constant power requirements [21]. As a result, the current technologies of renewable energy cannot balance industries like data centers, healthcare, food, and agriculture [22]. Further, the supply–demand gap in the global energy market is exacerbated by the mismatch between energy availability and consumption [23] and the fluctuating cost of energy during peak hours [24]. Renewable energy resources fluctuate on a daily and seasonal basis and are dependent, making it difficult to supply constant power, and being connected to the power grid creates challenges for maintaining grid voltage stability [25]. To address these problems, thermal energy storage (TES) is one of the most practical solutions when integrated with renewable energy sources, and is used to bridge the gap between energy generation and consumption, addressing discrepancies in time, temperature, power, or location [26]. TES employs materials with the capacity to capture thermal energy from daily atmospheric temperature flux between high and low points, enabling uniform energy storage and supply during both the charging and discharging phases [27]. Among TES, latent heat thermal energy storage (LHTES) technology is gaining prominence in energy-efficient and sustainable applications, particularly in improving energy efficiency [27].

Therefore, this review aims to comprehensively analyze the latest advancements in TES. The primary objectives of this review paper include providing an in-depth analysis of phase change material (PCM)-based TES, and their significance in building sectors. There are several studies available in the existing literature that have explored PV systems, HPs, and TES technologies individually, but there remains a lack of integrated analysis focusing on the potential of combining PV, HP, and PCM-based TES together as a whole system. This review paper contributes by bridging this gap, highlighting the significance of such integration and calling for future application of PV+HP+TES systems for the residential sector to reduce carbon footprints. This review also outlines future directions for deploying PV+HP+PCM-TES systems as a comprehensive solution for decarbonizing residential houses. This review summarizes the knowledge available in the literature on TES. Section 2 provides a detailed review of the classification, characterization, properties of TES, temperature range, and geometry of PCM containers and applications in various fields for PCM-TES. It also provides details about the challenges related to PCM-TES and the different modeling software for TES. Section 3 highlights the integration of PCM-based TES with renewable energy systems and focuses on the integration of PCM-based TES with solar PV, and HP for different applications and in the building sector. It also shows the research gap in the literature for integrated sustainable systems to supply heating, cooling, domestic hot water, and electrical loads. Section 4 discusses the overview of the successful integration of TES with solar PV and HP for supplying thermal loads of buildings and PCM-TES as an efficient and sustainable energy storage future solution for current and future energy-related challenges, and their integration with solar PV+HP for the decarbonization of the residential sector. Finally, conclusions are drawn, and future work is outlined in Section 5.

2. Background

2.1. Thermal Energy Storage

TES is described as the storage of heat and cold in a medium where the system is designed for heat injection and rejection to/from the medium [28]. There are active and passive TES systems. Active systems use forced convection heat transfer, which can be direct or indirect [29]. Passive systems are typically dual-medium systems, with the heat transfer fluid used solely for charging and discharging a solid material. The critical TES design factors include high energy density, efficient heat transfer, material stability, reversibility, minimal thermal losses, and ease of control [29]. The primary factor influencing the dynamics of charging and discharging in a TES system is thermal conductivity [30]. Lower thermal conductivity leads to a reduction in energy generation capacity [31]. Palacios et al. [32] highlighted the absence of standardized methods for measuring thermal conductivity and reviewed existing methods for characterizing TES materials. Their review covered steady-state (such as the heat flow meter and guarded hot plate) and transient methods. Other considerations for selecting TES materials encompass operation strategy, maximum load requirements, temperature and enthalpy drop, and system integration [30]. The different requirements and reasons for selecting TES materials and systems are shown in

Table 1. Different characteristics of TES materials and systems with their requirements.

Characteristics	Reasons	Requirements	Limitation
Technological	Enhances integration and efficiency by improved TES-heat transfer fluid (HTF) transfer.	Optimized operational strategy	Limited commercial growth [34]
Economical	Long-term affordability and reduced maintenance cost.	Low cost, long lifespan, and abundant availability	High initial cost [34]
Environmental	Supports sustainability and energy-efficient material.	Non-polluting	Some PCMs pose leakage issues and require more research [35]
Chemical	Maintains stable, safe, and compatible operations.	Long-term chemical stability	Some PCMs may experience supercooling, phase segregation [35]
Physical	Compact and stable system with minimum container stress	Low vapor pressure, high density, and minimum volume change	Liquid leakage [35]
Kinetic	Meets the heat transfer requirements of the recovery system.	Minimal or no subcooling	Require additives [35]
Thermal	Provides substantial sensible heat storage by optimizing heat transfer with minimal loss and compact volume.	Favorable phase equilibrium, good conductivity	Some PCMs have low thermal conductivity [36]

, which is adapted from [33] to justify how each characteristic is important in the selection and design of a reliable and efficient TES.

Thermal energy storage can be classified based on physical and chemical processes as shown in Figure 1. Based on chemical processes, thermochemical energy storage provides long-term capability and high energy densities as it uses reversible chemical reactions to absorb and release heat necessitating intricate chemical process management [37]. On the other hand, based on the physical process, TES is divided into sensible heat storage (SHS) and latent heat storage (LHS) whose governing equations are shown in Figure 1.

Table 2. Definition of the variables.

Variables	Definitions	Units
$Q$	Quantity of heat stored	Joules (J)
$T_i$	Initial temperature	Degree Celsius (°C)
$T_f$	Final temperature	Degree Celsius (°C)
$m$	Mass of heat storage medium	Kilogram (kg)
$C_p$	Specific heat	J/kgK
$C_{ap}$	Average specific heat between $T_i$ and $T_f$	J/kgK
$a_m$	Melted fraction	-
${∆H}_m$	Melting heat per unit mass	J/kg

shows the definition of the variables. SHS is the most widely used approach for storing heat energy [26]. It is considered the simplest and most straightforward heat storage technology [38]. Sensible heat involves the exchange of heat within a system without a change in phase, leading to a change in the temperature of the storage material. The SHS method, however, has some drawbacks, such as having a low energy density and experiencing thermal energy loss at any given temperature [39]. On the other hand, LHS utilizes phase changes in materials, such as paraffins and salts, for energy storage applications in heating, cooling, and power generation [22]. This process is based on the absorption and release of heat during phase transitions, with the efficiency influenced by factors like the type of storage material and the operating temperature [40]. Although LHS offers a higher energy density, a narrower temperature range for storing and releasing heat, and efficiency compared to SHS, it comes with higher costs and maintenance requirements [41,42,43]. The LHS is described in detail in the following sections.

2.1.1. Latent Heat Storage

LHTES systems utilize PCMs, which play a crucial role in improving energy efficiency, managing mismatches between supply and demand, and enhancing the performance of energy systems [45]. During phase changes, PCMs can store thermal energy at nearly constant temperatures, ensuring much higher energy storage density than sensible heat storage materials, making them ideal for storing latent heat [38,46]. PCMs with high latent heat capacities are prime candidates for thermal energy storage. High latent heat PCMs are more desirable for TES because they can store a greater amount of thermal energy. While water/ice is commonly used for TES, it has limitations, including a restricted operating range and a significant space requirement that complicates integration with air conditioning systems, whereas PCM-based TES offers distinct advantages, including higher energy density [27]. Water-based TES requires operational temperatures below the boiling point, or pressurization must be managed to prevent water from evaporating. PCMs, in contrast, can store heat within a narrow temperature range without requiring pressurization, thanks to the phase change between solid and liquid states [47]. Various methods for TES, i.e., SHS and thermochemical energy storage, have been developed, but PCMs stand out due to their ability to absorb, store, and release large amounts of energy per unit mass within a specific phase transition temperature range [48]. This characteristic makes PCMs widely used in numerous TES applications [49]. Energy savings can also be achieved by incorporating PCMs in heat recovery and solar energy systems [48]. The selection of the PCM depends on different factors for example an ideal PCM should fulfill several criteria concerning its thermophysical, kinetic, and chemical properties [42,50,51]. Some of these important criteria for PCM are summarized here:

(i)

Thermal Properties

○

Suitable phase-transition temperature: The PCM should match the operating temperature of the heating or cooling application.

○

High latent heat of transition: A high latent heat, especially on a volumetric basis, minimizes the physical size of the storage unit.

○

Good heat transfer: High thermal conductivity facilitates efficient charging and discharging of stored energy.

(ii)

Physical Properties

○

Favorable phase equilibrium: Stable phase changes during heating and cooling cycles are desirable.

○

High density: Allows for a more compact storage container, reducing space requirements.

○

Small volume change: Minimal volume expansion or contraction during phase transitions helps maintain structural integrity.

○

Low vapor pressure: Low vapor pressure at operating temperatures reduces the risk of containment issues.

(iii)

Kinetic Properties

○

No supercooling: The PCM should avoid supercooling, which can hinder proper heat extraction, especially if it exceeds 5–10 °C.

○

Sufficient crystallization rate: The PCM should crystallize efficiently to release stored heat effectively.

(iv)

Chemical Properties

○

Long-term chemical stability: The PCM should maintain stability over time without significant degradation.

○

Compatibility with construction materials: The PCM should not react with or corrode materials used in the storage system.

○

No toxicity: It must be safe for human health.

○

No fire hazard: It should be non-flammable and non-explosive to ensure safety.

(v)

Economic Considerations

○

Abundant: It should be readily available in large quantities.

○

Available: Easy access to the PCM in the market is crucial.

○

Cost-effective: It should be affordable for large-scale applications.

2.1.2. Classification of PCM

PCMs are classified into three main categories: organic, inorganic, and eutectic materials as shown in Figure 2. The chemical properties of PCM classification [30] and general properties [52] are shown in

Table 3. Chemical properties of PCMs as per their classification.

PCMs
Organic	Inorganic	Eutectic
No supercooling Available across a wide temperature range Physically and chemically stable High latent heat of fusion Ability to melt congruently Flammable Low thermal conductivity Low volumetric energy intensity	High latent heat of fusion High thermal conductivity Minimal volume change during phase transition Readily available Non-flammable Supercooling Causes corrosion Does not melt congruently High volumetric energy intensity	Distinct melting temperature High volumetric thermal storage capacity Elevated cost Limited availability of property data

and

Table 4. General properties of PCMs as per their classification.

Properties	Paraffins	Non-Paraffins	Metallics	Hydrates Salts
Latent Heat (kJ/kg)	200–280	90–250	25–100	60–300
Melting Temperature (°C)	−20–100	5–120	150–800	0–100
Thermal conductivity (W/mK)	0.2–0.4	0.15–0.25	20–200	0.5–1.0

, respectively. There are two forms of organic PCMs: paraffin and non-paraffin. These materials typically have congruent melting behavior, and self-nucleation capabilities, and are generally non-corrosive to container materials [51]. Organic PCMs commonly used for heating and cooling buildings operate in the temperature range and their melting points and latent heats of fusion are listed in

Table 5. Organic substances used as PCMs.

Compound	Melting Point (°C)	Heat of Fusion (kJ/kg.K)	Structural Characteristics	References
n-Tetradecane	5.8–6.0	227–229	High crystalline, sharp phase change	[53]
Caprylic acid	16.3	148	Enable hydrogen bonding	[54]
Butyl stearate	19	140	Low supercooling, good thermal stability	[55]
Dimethyl sabacate	21	120	Stable melting/freezing point	[56]
n-Heptadecan	22–22.6	164–214	Low thermal conductivity	[57]
Paraffin C13-C24	22–24	189	Chemical stability	[58]
Undecylenic acid	24.6	141	Increasing chemical reactivity	[59]
1-Dodecanol	26	200	Hydrogen bonding and moderate polarity	[55]
Vinyl stearate	27–39	122	Support phase change properties	[56]
n-Octadecane	28.0–28.4	200–244	Stable melting point	[53]
Capric acid	32	152.7	Suitable for low-temperature thermal storage	[60]
n-Pentacozane	53.5	238	Suffer low conductivity	[57]
n-Tricontane	65.4	252	Require conductivity enhancement	[57]
Arachidic acid	74.0	227	Moderate latent heat and melting	[54]
Amides	82	241	Low volume change	[50]
Granulated sugar	179	179	Decompose before melting point	[61]

.

Inorganic materials, including salt hydrates and metallics, are known for their high latent heat per unit mass and volume, for example, Tutton salts and Krokhnkite-like sodium calcium sulfates [62], making them cost-effective compared to organic compounds. Paraffins have a material cost of USD 20–40/kWh, whereas some materials of salt hydrate are available for under USD 2/kWh [63,64]. They are also non-flammable, but problems like supercooling and decomposition can have an impact on their phase transition characteristics [51]. The melting points and latent heat of the fusion of some common inorganic PCMs are summarized in

Table 6. Inorganic substances used as PCMs.

Compound	Melting Point (°C)	Heat of Fusion (kJ/kg.K)	Structural Characteristics	References
$\mathrm{L}\mathrm{i}\mathrm{C}\mathrm{l}{\mathrm{O}}_3·3{\mathrm{H}}_2\mathrm{O}$	8	253	Sharp melting transition	[65]
$\mathrm{K}\mathrm{F}·4{\mathrm{H}}_2\mathrm{O}$	18.5	231	High latent heat of hydration/dehydration	[58,65]
$\mathrm{M}\mathrm{n}({\mathrm{N}{\mathrm{O}}_3)}_2·6{\mathrm{H}}_2\mathrm{O}$	25.8	125.9	Stable crystalline structure	[66]
$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{l}}_2·6{\mathrm{H}}_2\mathrm{O}$	29	190.8	Well-defined crystalline structure	[67]
$\mathrm{L}\mathrm{i}\mathrm{N}{\mathrm{O}}_3·3{\mathrm{H}}_2\mathrm{O}$	30	296	High volumetric latent heat	[65]
${\mathrm{N}\mathrm{a}}_2{\mathrm{S}\mathrm{O}}_4·10{\mathrm{H}}_2\mathrm{O}$	32	251	High latent heat	[65]
${\mathrm{N}\mathrm{a}}_2{\mathrm{P}}_2{\mathrm{O}}_7·10{\mathrm{H}}_2\mathrm{O}$	70	184	High latent heat	[68]
$\left(\mathrm{N}{\mathrm{H}}_4\right)\mathrm{A}\mathrm{l}{\left(\mathrm{S}{\mathrm{O}}_4\right)}_2·12{\mathrm{H}}_2\mathrm{O}$	95	269	Sharp melting/freezing points	[68]
$\mathrm{A}\mathrm{l}{\mathrm{C}\mathrm{l}}_3$	192	280	High heat of hydration	[69]
$\mathrm{L}\mathrm{i}{\mathrm{N}\mathrm{O}}_3$	250	370	High enthalpy of dissolution	[69]
$\mathrm{N}{\mathrm{a}}_2{\mathrm{O}}_2$	360	314	Exothermic chemical reaction	[69]
$\mathrm{L}\mathrm{i}\mathrm{H}$	699	2678	High heat of reaction on hydrolysis	[69]
$\mathrm{K}\mathrm{F}$	857	452	High lattice energy	[70]
$\mathrm{M}\mathrm{g}{\mathrm{F}}_2$	1271	936	Strong lattice bond	[69]

.

A eutectic phase change material consists of two or more components that melt and freeze simultaneously, leading to the development of a crystalline mixture during the crystallization process [51]. For example, combining fatty acids with hydrated salt matrices can greatly minimize supercooling and phase separation while preserving high latent heat and cycle stability, according to recent research on fatty acid–hydrated salt composite PCMs [71].

Table 7. Eutectics substances used as PCMs.

Compounds	Melting Point (°C)	Heat of Fusion (kJ/kg.K)	Structural Characteristics	References
${\mathrm{H}}_2\mathrm{O}+\mathrm{p}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{a}\mathrm{c}\mathrm{r}\mathrm{y}\mathrm{l}\mathrm{a}\mathrm{m}\mathrm{i}\mathrm{d}\mathrm{e}$	0	292	Forms shape-stable hydrogel that prevents leakage	[72]
$45\%\mathrm{C}\mathrm{a}{\mathrm{C}\mathrm{l}}_2·6{\mathrm{H}}_2\mathrm{O}+55\%\mathrm{C}\mathrm{a}\mathrm{B}{\mathrm{r}}_2·6{\mathrm{H}}_2\mathrm{O}$	14.7	140	Enhance phase stability and latent heat capacity	[50]
$66.6\%\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{l}}_2·6{\mathrm{H}}_2\mathrm{O}+33.3\%\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2·6{\mathrm{H}}_2\mathrm{O}$	25	127	Promotes nucleation control	[58]
$48\%\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{l}}_2+4.3\%\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}+0.4\%\mathrm{K}\mathrm{C}\mathrm{l}+47.3\%{\mathrm{H}}_2\mathrm{O}$	26.8	188	Stable thermal cycling	[58]
$47\%\mathrm{C}\mathrm{a}({\mathrm{N}{\mathrm{O}}_3)}_2·4{\mathrm{H}}_2\mathrm{O}+53\%\mathrm{M}\mathrm{g}({\mathrm{N}{\mathrm{O}}_3)}_2·6{\mathrm{H}}_2\mathrm{O}$	30	136	Structural stability	[58]
$60\%\mathrm{N}\mathrm{a}\left(\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\right)·3{\mathrm{H}}_2\mathrm{O}+40\%\mathrm{C}\mathrm{O}({\mathrm{N}{\mathrm{H}}_2)}_2$	30	200.5	Reduce supercooling	[73]
$50\%\mathrm{N}\mathrm{a}\left(\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{O}\right)·3{\mathrm{H}}_2\mathrm{O}+50\%\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{N}{\mathrm{H}}_2$	40	255	Optimize melting behavior	[74]

shows a list of eutectic substances that can be used as PCMs.

2.1.3. Temperature Ranges for PCMs

The PCM must have a melting temperature that falls within the realistic application range to be chosen as the right PCM for any given application [33].

Table 8. Commercial manufacturer of PCMs.

Listed Number of PCMs	Temperature Range (°C)	Manufacturers
61	−114 to 164	Environmental Process Systems Ltd., Cambridgeshire, UK (epsltd.co.uk, accessed on 28 October 2024)
29	−3 to 100	RUBITHERM, Berlin, Germany (Rubitherm Technologies GmbH, accessed on 28 October 2024)
22	−50 to 78	TEAP (www.teappcm.com, accessed on 28 October 2024)
12	−33 to 27	Cristopia, Vence, France (www.cristopia.com, accessed on 28 October 2024)
9	−18 to 70	Climator, Skövde, Sweden (www.climator.com, accessed on 28 October 2024)
6	9.5 to 118	Mitsubishi Chemical, Tokyo, Japan (www.mfc.co.jp, accessed on 28 October 2024)
2	−22 to 28	Doerken, Herdecke, Germany (www.doerken.de, accessed on 28 October 2024)

shows the list of commercial manufacturers that produce over 100 PCMs, whereas

Table 9. Potential TES storage applications in different sectors with their temperature range.

Temperature Range (°C)		Sectors/Application	References
Minimum (°C)	Maximum (°C)
40	120	Desalination	[78,79]
−40	350	Heating and cooling
−40	−10	Cold production	[80,81]
18	28	Space heating and cooling of buildings	[82,83]
29	80	Heating and cooling of water	[82,84]
80	230	Absorption refrigeration	[85]
−60	350	Adsorption refrigeration	[86]
60	260	Industry	[87]
30	1600	Industrial waste heat recovery	[88,89]
20	565	Solar energy
60	250	Solar cooling	[90]
20	150	Solar energy storage	[90]
250	565	Solar power plant	[91,92]
−269	130	Thermal protection
25	45	Electronic device thermal	[93]
85	120	Chips thermal protection	[94]
5	45	Data centers thermal protection	[95]
−269	130	Spacecraft electronics thermal protection	[96]
−30	121	Food thermal protection	[97]
−30	22	Biomedical applications	[98]
−50	800	Transportation
−50	70	Cabin heating and refrigeration	[99,100]
30	80	Battery and electronic protection	[99,100]
55	800	Exhaust heat recovery	[99,100]

shows the temperature range of potential TESs in different sectors [33]. Different PCMs are also studied based on their temperature range and application areas. For example, for domestic heating/cooling paraffin, water/ice, stearic acid, and n-octadecane are most commonly used in the temperature range of 0–65 °C, for absorption cooling system erythritol, RT100 is generally used for a temperature range of 80–120 °C, and for application of storage in solar power plant PCM like $\mathrm{N}\mathrm{a}{\mathrm{N}\mathrm{O}}_3,\mathrm{K}{\mathrm{N}\mathrm{O}}_3,\mathrm{N}\mathrm{a}\mathrm{O}\mathrm{H},\mathrm{K}\mathrm{O}\mathrm{H},\mathrm{Z}\mathrm{n}{\mathrm{C}\mathrm{l}}_2$ are used for temperature ranges above 150 °C [43]. PCMs can be categorized based on their phase change temperatures into four groups: low temperature (below 0 °C to no more than 5 °C), low-middle temperature (from 5 °C to 40 °C), middle temperature (from 40 °C to 80 °C), and high temperature (above 80 °C) [75,76]. A more detailed classification of PCMs specifically was summarized by Du et al. [77], which is shown in

Table 10. Specified temperature range of PCMs for different application areas [43,77].

Specified Temperature Range of PCMs (−20 to 200 °C)	Applications
Low-temperature range (−20 to 5 °C)	Domestic refrigerator
	Commercial refrigerated products
Medium-low temperature (5 to 40 °C)	Free cooling
	Passive heating/cooling for building
	Air conditioning system
	Solar absorption chiller
	Radiative and evaporative cooling
Medium temperature range (40 to 80 °C)	Electric device
	Solar air heater
	Solar stills
	Solar domestic hot water
High-temperature range (80 to 200 °C)	On-site waste heat recovery
	Off-site waste heat recovery
	Solar absorption cooling
	Solar thermal electricity generation

.

Once the selection of PCM is performed based on the temperature range of the application, the next crucial factors to consider are the geometry of the PCM container and the thermal and geometric specifications required for the intended amount of PCM [43]. These factors play a significant role in determining the heat transfer characteristics within the PCM, ultimately affecting the melt time and the overall efficiency of the PCM storage system [43]. The most common geometry studied for PCMs is shown in

Table 11. Geometry of containers employed in the study of PCMs.

Geometry of PCMs Container	References
Spherical	[101,102]
Cylindrical (Shell and tube)	[103,104,105,106,107,108,109,110]
Rectangular/slab	[102,111,112,113,114,115,116,117,118,119]
Cylindrical (Concentric annulus)	[102,118,120,121,122,123,124,125,126,127,128,129,130,131]

.

2.2. Application of PCM-Based Thermal Energy Storage in Different Fields

TES is being used increasingly by industries for a variety of purposes, including waste heat recovery, air-conditioning systems [132,133], building thermal comfort systems [134,135], electronic thermal management [136], biomedical applications, food transportation [137], thermally regulated textiles [138,139] and agriculture industries [140]. SHS is the most cost-effective and widely used TES technology [141]. These systems can also be integrated with solar thermal technologies for continuous heat supply [34,132,133]. Water tanks, a low-cost and effective option, are used in solar-assisted heat pumps [142] and hybrid systems [143], operating across a wide temperature range. TES is a flexible technology that can capture and store waste heat for later use, allowing it to be efficiently integrated with other systems to optimize energy utilization [144].

PCMs are widely used in the construction industry to enhance cooling, improve indoor comfort, and manage peak load shifting [145]. For example, heat generation in electronic devices is a common by-product and must be effectively dissipated to prevent device failure [146]. The PCM-filled aluminum heat sink can function as a TES device, safeguarding electronic equipment from immediate failure by absorbing and dissipating excess heat [147]. It should be noted that paraffin-based PCMs can cool equipment by up to 85% and are effective heat-absorbing media that enhance the efficiency of electronic devices [148]. Yet, another feasible example is the development of a cost-effective commercial solar power plant coupled with solar collectors and PCM tank storage [149]. PCM-based TES is also used in building applications using solar thermal energy with PCM for room heating and cooling [49]. For this, a thin layer of PCM can be applied as a coating or a wall-board setup can be created to incorporate the PCM [150]. The analysis results show a reduction in wall surface temperature by 1 °C, with a corresponding decrease in room temperature of 2.7 °C, and a surface heat flow reduction of up to 40% [151]. On the other hand, in the HVAC system, water–water HP integrated with two thermal storage PCM tanks (one cold and another hot tank) to supply the heating and cooling energy demands of the residential house compared to the conventional functioning mode of HP results in energy savings of up to 19% [152]. Similarly, PCM materials are used in different biomedical applications. The World Health Organization is encouraging research for developing cost-effective methods for preserving heat-sensitive medications at safe temperatures for extended periods [153]. This can be achieved by utilizing the heat storage and properties of PCMs [154]. Similarly, some examples of the use of PCM in textiles are aerospace textiles, automatic textiles, medical textiles, quilts, pillows, mattress covers, beds, helmets, and sheets [138,155]. PCMs can also be effectively used in food transport as they show potential for maintaining temperatures in temperature-sensitive foods [22]. These materials can help minimize temperature fluctuations and maintain the low temperatures required for food, vaccines, or clinical trial samples during transportation [156,157].

PCM-Based TES Application in Buildings

Building sectors are responsible for nearly 40% of global energy demand [158]. Heating and cooling applications account for 60–70% of the total energy consumption in the building sector, with most of this energy coming from fossil fuels [159]. As a result, the development and implementation of TES-based low-carbon energy solutions represent one of the most promising pathways toward achieving a low-carbon economy [160]. Research is being carried out on TES to increase building comfort and decrease peak load and electricity demand [161]. PCMs are an efficient medium for TES because of their high heat storage capability and feasible melting and freezing temperatures [162]. There are three different ways to use PCMs for heating and cooling of buildings, which include integration of PCM in building walls, different building components, and heat and cold storage units [51]. One of the reasons for this approach being widely accepted in regions with extremely cold or hot climates areas is that PCMs in active and passive storage systems can minimize the variation between the electrical energy consumption during the day and the night which usually arises from varying demands for domestic heating and cooling [82,83,163,164,165,166,167]. Passive heating/cooling systems refer to technologies or design elements that regulate building temperatures without relying on active mechanical devices, utilizing minimal to no external energy [168]. Active storage is usually used for off-peak storage of thermal energy in buildings and for example floor heating systems coupled to PV.

Table 12. List of commercial PCMs used in building for TES.

PCMs	Product Type	Melting Point (°C)	Heat of Fusion (kJ/kg)
RT 20	Paraffin	22	172
Climsel C23	Salt hydrate	23	148
Climsel C24	Salt hydrate	24	216
RT 26	Paraffin	25	131
RT 25	Paraffin	26	232
STL 27	Salt hydrate	27	213
RT 30	Paraffin	28	206
TH 29	Salt hydrate	29	188
RT 32	Paraffin	31	130
Climsel C32	Salt hydrate	32	212

shows the list of commercially available PCMs in the international market for use in buildings such as TES.

Table 13. Application of PCMs in buildings.

PCM Type	Applications	References
Inorganic	Walls, roof	[169]
Organic	Panels, (aluminum and composite wallboards)	[170]
Paraffin wax	PCM integrated wall	[150]
Organic	PCM layer-external wall	[171]
LA-SA/A1203/C	Building envelope	[172]
Inorganic salts	Concrete block	[173]
DuPont PCM (from EnergyPlus)	Passive PCM wallboards	[174]
Inorganic salt hydrate	PCM-layer roof	[175]
PCM (from EnergyPlus)	Building envelope (inner layer)	[176]

shows the application of PCMs in building sectors [51]. There are certain challenges in using PCMs in different areas of applications and equipment, which are summarized in

Table 14. Challenges of using PCMs in certain areas as equipment.

Equipment	Challenges	Pros	Cons	References
Batteries	Melting point within the optimal operating temperature range Density changes to prevent issues with the storage tank.	Improved temperature uniformity Lower peak temperatures Decreased system size	Heat accumulation Extra weight Undesirable thermal inertia	[177,178]
Solar PV	Latent heat should be maximized to reduce the physical size of the heat storage system.	Slows temperature increase. Minimizes loss of electrical efficiency Low vapor pressure	Undesirable thermal inertia	[179]
Solar heating system	Melting point of the storage material Mass of the PCM Phase separation in the PCM	Heat is absorbed and released at a constant temperature. PCMs are applicable in both active and passive heating and cooling systems. Stored energy can be accessed when the energy supply is unavailable. Lowers peak. energy demand. PCMs address the issue of sunlight’s temporal unavailability. Contributes to a positive environmental impact.	Low thermal conductivity Slow heat diffusion Decreased storage capacity. PCM must retain its original structure and composition. Resistance to various external factors	[180]
Air conditioning	Low heat transfer affects PCM charging time. Thermo-physical properties and encapsulation geometry of PCMs need optimization. Efficient charging and discharging of PCM are crucial.	Reduced Carbon emission Address fluctuations in solar energy availability. Provides a defrosting solution for air-conditioning systems.	Low heat transfer between HTF and PCM Implementation cost	[181,182]

, along with their advantages and disadvantages.

2.3. Modelling of PCM-Based TES

When it comes to the detailed modeling of TES, a numerical solution is used; however, at the same time, the most common problem encountered in modeling LHTES is the poor conductivity of PCMs [50]. A method of improving the effective heat transfer area during phase change is to closely space the sidewalls of the PCM container, which is made of a highly thermally conductive material; however, the moving boundary where heat and mass balance conditions must be met, which creates problems that are difficult to solve [50]. Numerical methods, including both finite difference [183,184] and finite element approaches [185,186], have proven to be more effective in solving the moving boundary problem. In the existing literature, computational tools like COMSOL Multiphysics [187], ANSYS FLUENT [188], OpenFoam [189], and Star-CCM+ [190] are used for simulations and analyses of phase change phenomena. These software tools offer the ability to simulate systems that involve phase change processes within a defined temperature range [39]. Dahash et al. [191] classified these tools into three main types:

(i)

Building physics envelopes heat and mass transfer

E.g., WUFI [192], Delphin [193]

(ii)

Computational fluid dynamics software

E.g., ANSYS Fluent [188], OpenFOAM [189], COMSOL Multiphysics [187]

(iii)

Energy system simulation software

E.g., MATLAB/Simulink [194], TRNSYS [195], Dymola [196]

Table 15. Software tools and work performed in TES.

Overview of Work Performed on TES	Software Used	References
Developed approximation to investigate natural convection inside PCM using a conduction–convection model. Governing equations were solved using finite element simulation.	COMSOL Multiphysics	[206]
Presented a detailed numerical analysis to demonstrate the behavior of heat transfer PCM-TES.	COMSOL Multiphysics	[207]
Provided a simplified dynamic model, showing that the system may be limited to configurations with seasonal heat storage due to model complexity.	Aspen HYSYS	[208]
Simulated heat transfer problems during solidification and melting. Modeled an innovative pipe-embedded ventilation roof with an outer-layer PCM.	FLUENT	[209]
Coupled 3D transient numerical simulation with global sensitivity analysis to study the relationship between input parameters and output indicators during the charging phase, accessing how supply temperature affects system efficiency.	Numerical simulation	[210]
Developed and simulated an aquifer TES for district heating/cooling to evaluate performance over different time periods.	MODFLOW	[211]

shows some of the work performed using the software tools on the TES. Modeling buildings in computer simulation software to optimize the energy requirement of buildings is performed apart from field testing and numerical analysis. Energy simulation software is an important tool for researchers to analyze energy requirements [38]. Researchers use this to validate their experimental results [197]. There are several simulation tools available commercially that are used for the optimization of energy requirements [198]; among them, EnergyPlus [199], TRNSYS [195], and ESP-r [200] are the most commonly used simulation tools [201,202]. Modern techniques also include machine learning, such as artificial neural networks and genetic and support vector machines that are used to model and optimize PCM performance in building applications [203]. These software packages have notable features that can analyze the thermal performance of a building, and these features [49] are summarized in

Table 16. Characteristics features of building energy simulation software.

Characteristic Features	EnergyPlus [212]	ESP-r [213]	TRNSYS v.18
Open-source Software	Y	Y	L
Price (USD)	Free	Free	2620
Automatic calculation for dew point temperature	Y	N	N
Automatic calculation for dry bulb temperature	Y	N	N
Controlled window operating	Y	Y	O
Displacement ventilation	Y	Y	O
Hybrid natural and mechanical ventilation	N	O	O
Idealized HVAC system	Y	Y	Y
Inside radiation view factors	Y	Y	N
Internal thermal mass	Y	Y	Y
Multizone airflow	Y	Y	O
Natural ventilation	Y	Y	O
Solar gain and daylighting	Y	Y	Y
User configuration HVAC system	Y	Y	Y

Y: yes available; L: licensed; N: not available; O: optional.

. There are, however, several challenges when it comes to modeling PCM-based TESs which include the hysteresis effect, i.e., the difference between melting and solidification process, which can significantly affect the simulation results [204]. The PCM may not fully melt or solidify in real-life applications which poses a challenge for modeling partial phase transition [204,205].

3. PCM-Based TES Integration with Renewable and Mechanical Systems

This section puts together the existing literature on the integration of PCM-TES with renewable and mechanical systems separately, with a focus on building use and highlights the pros and cons. Section 3.1 starts with the integration of PV+PCM and overall application and summarizes it in Table 17, while additionally focusing on the PV+PCM application in the building. Similarly, in Section 3.2, the HP basics are given and HP+PCM application in the building sectors is summarized in Table 18. Lastly, in Section 3.3, the benefits and shortcomings of the PV+HP system are mentioned, highlighting the research gap and calling for the integration of TES and PCM-TES with existing systems in building sectors as a sustainable solution.

3.1. Integration of Solar Photovoltaics (PV) with PCM-TES

To attain the UN’s Sustainable Development Goals (SDGs) and lessen dependency on fossil fuels, many countries have proposed a variety of renewable energy-producing initiatives [214]. Solar energy plays an important role in achieving energy sustainability and NetZero energy goals [215]. Geographical location plays an important role in the performance of solar PV [216]. In the case of solar PV, tropical regions are blessed with large solar fluxes [216] and can be effectively used to deploy large-scale PV plants without disturbing the habitat [217]. At the same time, an inherent challenge of PV system performance is the negative sensitivity to operation temperature [218], resulting in a loss in efficiency, especially in hot regions/seasons [219]. Within a solar photovoltaic cell, only about 20% of the incident solar radiation is converted to electrical energy, with the remainder being converted to heat [220]. The surplus heat raises the operating temperature of PV cells, affecting the silicon layer of the cells, which leads to decreased efficiency, reduced power output [221,222] and a shorter lifespan of the cells [223]. An increase in temperature by 1 °C can result in a 0.65–0.85% decrease in power output [224]. The summer months can drive PV cell temperatures up to 40–70 °C, which causes a significant 7.5–22.5% decrease in conversion efficiency [225].

A solution to this challenge is PV cooling techniques to regulate the temperature of the PV panels [226]. Solar PV cooling can be divided into active and passive cooling techniques [227]. Active cooling strategies, commonly using forced air or water, require electrical power input in various forms for implementation [228]. In tropical regions, solar PV cooling strategy with forced air circulation through aluminum fins has achieved a relative efficiency improvement of approximately 15% [229]. Numerical simulation models of cooling techniques, incorporating active air and water as cooling fluids, were developed in [230] to help identify the most effective cooling system for maximizing benefits before experimental setups. Other examples of active cooling include using flowing water on the PV module surface [231,232], cooling medium flowing through channels [233,234], nanofluids [235], microchannels [140,236], and jet impingement [237,238]. Although active cooling systems improve solar PV performance, they have limitations such as frequent maintenance, equipment replacements, continuous power consumption, and increased installation and operating costs, which extend the payback period [228].

While passive cooling does not require power inputs, it does need additional infrastructure to achieve cooling, and it uses natural water and air cooling, which requires no extra setup. These systems typically yield lower performance and can sometimes have adverse effects [228]. For example, agrivoltaics are a natural cooling strategy comprising crops beneath the panel roofs [239]. Agrivoltaics significantly reduce the temperature of solar panels, leading to enhanced performance [240]. Agrivoltaics face limitations such as the need for location selection, additional structural setups for improved yield, and the constraints of growing only specific crops [241]. The water-based PV consisting of floating photovoltaics (FPV) is an effective way of passive cooling [242]. FPV has been widely installed over the water surface and has advantages like no land acquisition, reduced water evaporation, water quality improvement, decreased operating temperature, and protection from excessive algae growth [243].

Another example of passive cooling is the use of PCM. PCM is an effective coolant that can be directly used in cooling applications through a simple embedding technique [244]. PCM can be directly applied to the rear side of PV panels or enclosed within small conductive containers or packets [245]. Additionally, PV-PCMs can be arranged without direct contact using radiative cooling methods [246]. These research studies have advocated the use of PCM for PV panel cooling [247,248,249,250,251,252,253,254]. Table 17 shows the summary of some of these studies for PV-PCM integration.

Table 17. Various studies related to PV-PCM integration.

PCM Type	Method of Analysis	System Configuration	Location/ Year	Results	Reference
n-Octadecane	Numerical + experimental	PV/PCM/Thermoelectric generator (TEG)	Laboratory/2010	- Integrated PV/TEG/PCM system achieved ~9.5% higher power output compared to standard PV and PV/TEG systems during the initial 1.5-h period. - 50 mm PCM with 5 W/m·K thermal conductivity, 40–45 °C phase change provided optimal thermal performance.	[255]
OM 29	Experimental	PV-PCM+ aluminum plate	Chennai, India/2019	- PV-PCM panel with aluminum sheet increased conversion efficiency by 24.4%. - It showed a temperature decrease of 10.35 °C, leading to 2% increase in electrical efficiency. - FLIR thermal images revealed maximum temperature reduction of 13 °C on Day 1, 7.7 °C on Day 2 for the PV-PCM system. - PV-PCM system with aluminum demonstrated a 30% increase in electrical power output compared to the uncooled PV panel.	[256]
Paraffin wax	Experimental	PV module, PV-PCM, PV-PCM-thermal	Shanghai, China/2019	- PV-PCM system reduced PV module temperature by 23 °C compared to standard PV panel. Temperature reduction resulted in a 5.18% increase in electricity output.	[248]
Paraffin wax	Experimental + numerical	PCM matrix placed 6mm below the panel	Phitsanulok, Thailand/2020	- PV modules with composite PCM reduce temperature to 47.81 °C, lowering it by 6.7 °C compared to a conventional PV module. - Maximum efficiency achieved was 14.75% with the composite PCM matrix at an optimal thickness of 2.5 cm. - Composite PCM enhanced the PV power conversion efficiency by 0.46%.	[257]
(i) 82wt% coconut oil + 18wt% sunflower oil. (ii)(i) + Boehmite nano powder	Experimental	Modified geometry of PCM container: PV, PV+composed oil PCM, PV + nano-composed oil	Laboratory/2020	- Compared to reference case, the maximum efficiency increase achieved with composite oil and the nano-enhanced composite oil PCM configurations was 29.24% and 48.23%, respectively, at a radiation intensity of 690 W/m2.	[258]
Refined paraffin wax	Numerical +experimental	PCM-embedded PV panel	Laboratory/2021	- Four tilt angles (0°, 30°, 60°, and 90°) were examined. - As the tilt angle increases from 0° to 90°, the melting time of PCM decreases, improving cooling performance. - Increasing the tilt angle enhances convection heat transfer, accelerating the melting process of PCM. - PV cell temperature is reduced by 0.4% to 12% as the tilt angle increases from 0° to 90° due to improved natural convection.	[224]
Paraffin RT27	Numerical + experimental	PCM encases with PV panel	Chania, Greece/2023	- Cooling with PCMs increased energy output by 9.4%, but system required high initial energy investment, resulting in a lower energy return-on-investment (1.79 for PV-PCM vs. 4.94 for PV) and a longer energy payback time (~14 years for PV-PCM vs. ~5 years for PV).	[259]
HS 29	Experimental	PV-PCM embedded with fins placed in still water	Madurai, India/2024	- Proposed system increased daily power output by 8.12% and 9.39% compared to uncooled panels over two observed days. Maximum power enhancement reached 20.25%.	[228]

Table 17. Various studies related to PV-PCM integration.

PCM Type	Method of Analysis	System Configuration	Location/ Year	Results	Reference
n-Octadecane	Numerical + experimental	PV/PCM/Thermoelectric generator (TEG)	Laboratory/2010	- Integrated PV/TEG/PCM system achieved ~9.5% higher power output compared to standard PV and PV/TEG systems during the initial 1.5-h period. - 50 mm PCM with 5 W/m·K thermal conductivity, 40–45 °C phase change provided optimal thermal performance.	[255]
OM 29	Experimental	PV-PCM+ aluminum plate	Chennai, India/2019	- PV-PCM panel with aluminum sheet increased conversion efficiency by 24.4%. - It showed a temperature decrease of 10.35 °C, leading to 2% increase in electrical efficiency. - FLIR thermal images revealed maximum temperature reduction of 13 °C on Day 1, 7.7 °C on Day 2 for the PV-PCM system. - PV-PCM system with aluminum demonstrated a 30% increase in electrical power output compared to the uncooled PV panel.	[256]
Paraffin wax	Experimental	PV module, PV-PCM, PV-PCM-thermal	Shanghai, China/2019	- PV-PCM system reduced PV module temperature by 23 °C compared to standard PV panel. Temperature reduction resulted in a 5.18% increase in electricity output.	[248]
Paraffin wax	Experimental + numerical	PCM matrix placed 6mm below the panel	Phitsanulok, Thailand/2020	- PV modules with composite PCM reduce temperature to 47.81 °C, lowering it by 6.7 °C compared to a conventional PV module. - Maximum efficiency achieved was 14.75% with the composite PCM matrix at an optimal thickness of 2.5 cm. - Composite PCM enhanced the PV power conversion efficiency by 0.46%.	[257]
(i) 82wt% coconut oil + 18wt% sunflower oil. (ii)(i) + Boehmite nano powder	Experimental	Modified geometry of PCM container: PV, PV+composed oil PCM, PV + nano-composed oil	Laboratory/2020	- Compared to reference case, the maximum efficiency increase achieved with composite oil and the nano-enhanced composite oil PCM configurations was 29.24% and 48.23%, respectively, at a radiation intensity of 690 W/m2.	[258]
Refined paraffin wax	Numerical +experimental	PCM-embedded PV panel	Laboratory/2021	- Four tilt angles (0°, 30°, 60°, and 90°) were examined. - As the tilt angle increases from 0° to 90°, the melting time of PCM decreases, improving cooling performance. - Increasing the tilt angle enhances convection heat transfer, accelerating the melting process of PCM. - PV cell temperature is reduced by 0.4% to 12% as the tilt angle increases from 0° to 90° due to improved natural convection.	[224]
Paraffin RT27	Numerical + experimental	PCM encases with PV panel	Chania, Greece/2023	- Cooling with PCMs increased energy output by 9.4%, but system required high initial energy investment, resulting in a lower energy return-on-investment (1.79 for PV-PCM vs. 4.94 for PV) and a longer energy payback time (~14 years for PV-PCM vs. ~5 years for PV).	[259]
HS 29	Experimental	PV-PCM embedded with fins placed in still water	Madurai, India/2024	- Proposed system increased daily power output by 8.12% and 9.39% compared to uncooled panels over two observed days. Maximum power enhancement reached 20.25%.	[228]

Another application of PV-PCM integration includes heating the building where the building envelope is used as the TES. Zhi et al. [260] presented an experimental investigation with a controller-less PV heating system using the building envelope for thermal storage in a farmhouse in northern China. The study shows the system, comprising insulation boards, electric heating wires, and a thermal storage wall, that absorbs and releases heat after being fed by PV electricity. Results showed that indoor temperatures exceeded 16 °C for 60% of the heating season, with averages of 21.7 °C on sunny days and 14.2 °C on rainy days. The thermal storage wall reduced indoor temperature fluctuations by 47% and improved supply–demand matching by 77%. It also supplied 58% of heating energy during PV power shortages. With a unit heating cost of 1094 CNY and a 6.5-year payback, the system is a cost-effective alternative to traditional energy storage methods. Some of the studies that show the PV-PCM applications in building envelopes are [261,262,263,264,265].

Similarly, Tang et al. [266] proposed an energy-efficient PV-PCM window, which converts solar energy into electricity and stores excess heat in a modular PCM for nighttime release. The modular PCM design allows for easy adjustment to suit varying weather conditions. Compared to clear glazing, PV, and PCM windows, the PV-PCM window reduced peak indoor temperatures by up to 10.94 °C and lowered indoor heat gains by 265.96–364.97 kJ. The SHGC and U-value were reduced to <0.30 and <2.50 W/(m²·K), respectively, while improving power generation due to the PCM’s cooling effect. The PV-PCM window offers enhanced thermal regulation, flexibility, and energy efficiency, contributing to building sustainability. PCMs can be integrated into different parts of the window system, such as the cavity between glazing panes, window shutters or louvers, and frames [267,268,269]. Some of the recent studies that show the PV-PCM integration in windows for saving in building energy consumption are [270,271,272,273,274].

Most interestingly, Szajding et al. [275] proposed a way for integrating a 9.6 kWp solar PV system with 3kWh PCM-TES such that the PCM is charged with electric heaters via the PV system to supply the heating and DHW demand of the house, which is a single detached, ground floor house located in central Poland. The TES, designed with a modular PCM unit and equipped with thin aluminum fins to enhance heat transfer, uses an electric heater for charging, and water is used as heat transfer fluid for discharge. The design is validated by numerical simulations, optimizing heat distribution and charging time. The result shows that the integration with a PV system demonstrated that the TES efficiently stores excess energy, supporting DHW and central heating, and reducing reliance on natural gas by up to 71% during the heating season. Increasing the heater temperature from 60 °C to 80 °C improved the storage capacity by 12%, from 253 kJ/kg to 287 kJ/kg, and reduced PCM melting time by 67%.

3.2. Integration of HP with PCM-TES

HP is a mechanical technology that, when driven with renewable electricity, can potentially substitute traditional heating systems such as heaters, boilers, and furnaces that rely on fossil fuels [276]. HP can play a major role in cutting carbon dioxide emissions as humanity works toward becoming carbon-neutral [277]. HP can be divided based on the following criteria [278]:

(i)

Based on driving energy: Electric-driven, heat-driven

(ii)

Based on thermal sources: Air source HP (ASHP), water source HP (WSHP), groundwater source HP (GSHP), solar-assisted HP (SAHP), and waste heat source HP (WHSHP).

(iii)

Based on temperature range: Low temperature (<60 °C), medium temperature (60–100 °C), high temperature (100–200 °C), and ultra-high temperature (>200 °C)

ASHP efficiency is greatly affected by the air temperature, which makes it hard to perform well during severely cold climates [279]. On the other hand, GSHP has higher efficiency and is more stable compared to ASHP but, at the same time, is expensive [280]. Recently, low and medium-temperature HPs are most widely used in buildings, whereas with the development of high and ultra-high-temperature HPs, their use will not only be limited to supplying space and water heating but higher-grade can be used in many industrial fields [281].

HP can reduce a home’s primary energy usage by 15–50% compared to traditional systems such as condensing boilers or furnaces [282]. On the other hand, using HP for energy supplies often creates an imbalance between the supply and demand of the grid [283] as the operation of large numbers of HP in cold climate regions creates a huge peak load on the grid [284]. A sustainable solution to this could be the use of TES and demand response, which could reduce peak demand, power generation requirement [285], power losses, and economic cost [286]. Among TES, PCM has received more attention recently [287,288] because of its higher energy storage density and charge and the discharge temperature being close to isothermal in phase change interval [289]. These qualities of PCM make it easy to integrate with HP [290]. PCM can be applied to HP on the condenser side [291], evaporator side [292], desuperheater (auxiliary heat exchanger that uses superheated gases from the HP compressor to heat water) [293], heat transfer fluid [294], and the thermal buffer tanks of HP heating [295,296]. Table 18 shows the summary of different PCMs used in integration with HP, with the software used and the results obtained.

Table 18. Summary of HP integrated with PCM.

PCM Type	HP Type	Software Simulation	Results	Refs.
Paraffin	Air–Water HP	Numerical	COP increased by 13.3% at an inlet water temperature of 30 °C. Reduce return water temperature.	[295]
Paraffin	Low-temperature ASHP	Numerical	Optimization of spherical heat storage to enhance the efficiency of energy storage in HP. Centre of connection for cascade HP	[297]
Modified CH3COONa·3H2O Composite PCM (CPCM)	ASHP	Experimental	Sodium acetate trihydrate (SAT)-based CPCM melting point developed to enhance HP performance.	[298]
Modified CH3COONa·3H2O	Hot water HP	Experimental	Obtain a suitable phase change temperature range with SAT-based CPCM.	[299]
Lauric acid	GSHP	Numerical	TES can offset peak demand and reduce volume using a PCM-incorporated water tank.	[300]
Palmitic acid	HP	Numerical+ TRNSYS	Analytic solution to determine optimal encapsulation thickness (0.8–3 cm) of PCM in a hybrid water–PCM tank charged by HP. Small temperature difference allows thinner encapsulation while maintaining a constant charging rate.	[301]
PCM	ASHP	TRNSYS	Transient simulation analysis for the PCM floor connected with HP. PCM flooring improves temperature control in winter while mitigating excessive heat in high-efficiency buildings.	[302]
PCM	GSHP	Numerical	Thermophysical properties of soil altered by different quantities of microencapsulated PCM. Indicated PCM-modified soil exhibits a temperature reduction of 3 °C compared to traditional soil. HP COP increasing by over 17%.	[303]
PCM	Horizontal GSHP	TRNSYS	Shows the different cases of PCM in-floor compared to radiant floor heating. Case 1 shows: - Indoor temperature stability increases by 8.6%. - Load flexibility enhanced by 18.1%. Total operating cost reduction by 13.8%.	[304]
Paraffin	ASHP	MATLAB+TRNSYS	Analysis for investigation of energy performance. For a specified stored heat energy demand, the relationship between water mass flow rate and total energy consumption is represented by a quadratic equation.	[305]
Paraffin	ASHP	Experimental	PCM-enhanced storage tank increased heat storage capacity by 14% with minimal increase in tank volume.	[306]
Stearic acid–graphite	HP	Numerical+MATLAB-Simulink	DHW production using integrated HP+PCM-based TES with three operation cases. HP runs on a constant heat source, solar PV, and cheap electricity.	[47]
Paraffin/fatty alcohol	Geothermal HP	Experimental	LHTES+geothermal HP tested for DHW application. Results exhibited stability of both PCMs at low heating and cooling rates (0.1–0.8 K/min). The hot water temperature produced is above 40 °C.	[307]
Paraffin	ASHP	Mathematical	Controlling the charging time is crucial to keep the PCM temperature below the phase change endpoint to prevent system damage. COP of system decreases from 2.01 to 1.97 with a decreasing rate of 1.9% during the charging process indicating stable operation.	[308]
Paraffin	ASHP	Experimental	LHS + condenser of ASHP with an average COP variation between 2.19 and 2.34.	[309]
Expanded graphite/paraffin (EGP)	ASHP water heater	Experimental	Discharging performance of the EGP heat exchanger (HX) to ascertain if the rapid discharging of the PCM-HX can heat the cold water to the required temperature of DHW. EGPHX maximum expansion rate of 16 L was 6.25% at a temperature of 73.5 °C.	[310]
Paraffin	ASHP	Experimental + Mathematical	Integrates ASHP +LTES unit based on a condensing unit and specially designed HX to improve system performance in cold regions. Small and large pitch fin thickness accelerates the melting of the PCM. Three-stage PCM improves energy and exergy performance compared to single-stage PCM with COP improvement reaching 4.01% and overall exergy efficiency increasing by 4.65%.	[311]
Enhanced acid	GSHP	Numerical	PCM is proposed as a backfill material to improve the performance of GSHP. Four backfill materials include soil, paraffin RT27, acid and enhanced acid. Results show PCM is advantageous over soil because of thermal effect and consistent temperature of PCM.	[312]

Table 18. Summary of HP integrated with PCM.

PCM Type	HP Type	Software Simulation	Results	Refs.
Paraffin	Air–Water HP	Numerical	COP increased by 13.3% at an inlet water temperature of 30 °C. Reduce return water temperature.	[295]
Paraffin	Low-temperature ASHP	Numerical	Optimization of spherical heat storage to enhance the efficiency of energy storage in HP. Centre of connection for cascade HP	[297]
Modified CH3COONa·3H2O Composite PCM (CPCM)	ASHP	Experimental	Sodium acetate trihydrate (SAT)-based CPCM melting point developed to enhance HP performance.	[298]
Modified CH3COONa·3H2O	Hot water HP	Experimental	Obtain a suitable phase change temperature range with SAT-based CPCM.	[299]
Lauric acid	GSHP	Numerical	TES can offset peak demand and reduce volume using a PCM-incorporated water tank.	[300]
Palmitic acid	HP	Numerical+ TRNSYS	Analytic solution to determine optimal encapsulation thickness (0.8–3 cm) of PCM in a hybrid water–PCM tank charged by HP. Small temperature difference allows thinner encapsulation while maintaining a constant charging rate.	[301]
PCM	ASHP	TRNSYS	Transient simulation analysis for the PCM floor connected with HP. PCM flooring improves temperature control in winter while mitigating excessive heat in high-efficiency buildings.	[302]
PCM	GSHP	Numerical	Thermophysical properties of soil altered by different quantities of microencapsulated PCM. Indicated PCM-modified soil exhibits a temperature reduction of 3 °C compared to traditional soil. HP COP increasing by over 17%.	[303]
PCM	Horizontal GSHP	TRNSYS	Shows the different cases of PCM in-floor compared to radiant floor heating. Case 1 shows: - Indoor temperature stability increases by 8.6%. - Load flexibility enhanced by 18.1%. Total operating cost reduction by 13.8%.	[304]
Paraffin	ASHP	MATLAB+TRNSYS	Analysis for investigation of energy performance. For a specified stored heat energy demand, the relationship between water mass flow rate and total energy consumption is represented by a quadratic equation.	[305]
Paraffin	ASHP	Experimental	PCM-enhanced storage tank increased heat storage capacity by 14% with minimal increase in tank volume.	[306]
Stearic acid–graphite	HP	Numerical+MATLAB-Simulink	DHW production using integrated HP+PCM-based TES with three operation cases. HP runs on a constant heat source, solar PV, and cheap electricity.	[47]
Paraffin/fatty alcohol	Geothermal HP	Experimental	LHTES+geothermal HP tested for DHW application. Results exhibited stability of both PCMs at low heating and cooling rates (0.1–0.8 K/min). The hot water temperature produced is above 40 °C.	[307]
Paraffin	ASHP	Mathematical	Controlling the charging time is crucial to keep the PCM temperature below the phase change endpoint to prevent system damage. COP of system decreases from 2.01 to 1.97 with a decreasing rate of 1.9% during the charging process indicating stable operation.	[308]
Paraffin	ASHP	Experimental	LHS + condenser of ASHP with an average COP variation between 2.19 and 2.34.	[309]
Expanded graphite/paraffin (EGP)	ASHP water heater	Experimental	Discharging performance of the EGP heat exchanger (HX) to ascertain if the rapid discharging of the PCM-HX can heat the cold water to the required temperature of DHW. EGPHX maximum expansion rate of 16 L was 6.25% at a temperature of 73.5 °C.	[310]
Paraffin	ASHP	Experimental + Mathematical	Integrates ASHP +LTES unit based on a condensing unit and specially designed HX to improve system performance in cold regions. Small and large pitch fin thickness accelerates the melting of the PCM. Three-stage PCM improves energy and exergy performance compared to single-stage PCM with COP improvement reaching 4.01% and overall exergy efficiency increasing by 4.65%.	[311]
Enhanced acid	GSHP	Numerical	PCM is proposed as a backfill material to improve the performance of GSHP. Four backfill materials include soil, paraffin RT27, acid and enhanced acid. Results show PCM is advantageous over soil because of thermal effect and consistent temperature of PCM.	[312]

3.3. Integration of Solar PV, HP, and PCM-TES

When it comes to the benefits of heat pumps, they are renowned for their high efficiency and environmental sustainability and when integrated with residential solar PV systems they enhance solar energy self-consumption and facilitate the possibility of off-grid operation [313,314,315]. PV self-consumption and self-sufficiency remain at 24% and 52%, respectively, because peak solar PV generation and home electrical do not always coincide [316]. On the other hand, when PV is integrated with HP, self-consumption and self-sufficiency can reach up to 68% and 50%, respectively [20]. Moreover, some solar homeowners have seen the value of their investments degraded because their excess PV energy-fed grid was devalued from reduced feed-in tariffs [317], and manipulation of rate structures [318]. In such scenarios, heat pumps present a promising solution for increasing the solar fraction of residential loads, as they can utilize excess PV energy to generate heating and cooling for space conditioning and DHW [313,314,315]. Traditionally, this energy can be stored efficiently in an electric battery, ensuring that surplus solar energy is consumed during the day rather than being wasted. Researchers have extensively explored the use of electric battery storage to boost PV self-consumption [319,320,321,322], but the high capital costs of electrical batteries yield a poor return on investment compared to TES [323,324,325] and are challenged by environmental toxicity [326,327]. Thygesen and Karlsson [328] conducted a direct comparison between battery storage and TES, which concluded that the levelized cost of electricity for the battery system is twice that of the TES option.

The mismatch between energy production and consumption can also be addressed through TES, which allows excess renewable energy to be stored until periods of high heat demand [329]. The optimized selection of HP, PV systems, and thermal storage sizing, combined with simple controls targeting periods of PV availability, has been shown to reduce domestic grid consumption by up to 80% [330]. Several scholars have studied the impact of integrating PV systems with HP and TES for residential service systems. Some studies used a PV+HP system along with a hot/cold water tank [152,331,332,333,334]. While water is a competitive storage option for PCM, as it is very low cost and easy to utilize, the PCM-based TES has a higher energy density [335]. Additionally, water-based TES requires operating temperatures to remain below the boiling point or necessitates pressurization to prevent evaporation, whereas PCM can store heat within a narrow temperature range without the need for pressurization, as the phase change occurs between solid and liquid states [47]. PCM-based TES is separately studied in [336,337,338,339], but when it comes to the integration of PV+HP system with PCM-TES to supply both thermal and electrical load of residential houses, there are no studies available in the literature. The studies available do not focus on the whole PV+HP+PCM-TES system together. Although PV+HP shows promising results and PCM-TES are studied showing easy, cheap, and sustainable solutions, there is a gap in the literature when it comes to integrating them all together for the decarbonization of the residential sector.

Inkeri et al. [47] presented the numerical modeling of an HP integrated with shell-and-tube PCM-TES using a stearic acid and graphite mixture as PCM to supply only the DHW daily demand for a building with several apartments. The study presented the three cases in which the HP pump is run at a constant heat source, solar electricity, and cheap electricity. The result shows that for a daily DHW demand of 138.4 kWh, the HP output required is approximately 58% more for cheap electricity compared to solar electricity. The limitation of this study is it does not include the heating and cooling demands of the house. The study focuses on the integration of HP and PCM-TES and does not provide details on PV integration.

4. Discussion

The building sector accounts for a significant portion of the world’s energy demand [340]. In the European Union, the building sector accounts for 40% of energy consumption [341], and about 35% of greenhouse gas emissions [342]. The Canadian residential sector consumes almost 17% of the total energy in the country [343]. In the United States, the building sector consumes 40% of the total energy and is responsible for 75% of U.S. electricity consumption [344] and about 35% of carbon dioxide emissions [344]. Overall, the building sector for the whole world is responsible for 30% of global final energy consumption and 26% of global energy-related emissions [345]. In buildings, almost half of the global energy demand is due to space and water heating, and two-thirds of this heating energy use still relies on fossil-fueled furnaces or boilers (gas, oil, or even coal) [345]. Hence, in addition to providing electricity from renewable sources, decarbonization in buildings requires the replacement of combustion appliances with green electric ones.

The integration of solar PV with HP and PCM-TES to supply the residential electrical and thermal loads can be one of the sustainable future solutions to achieve the energy goal. This sustainable integration includes renewable energy sources for electricity in the residential sector, HP replacing conventional heating systems, and PCM-based TES to store the energy for later use. The integration of either of these two systems is available in the literature and, as presented in this review in Section 3.1 and Section 3.2, has shown significant results. Figure 3 shows methods discussed in the literature to reduce the fossil fuel use in buildings to provide both electricity and thermal control using an integration of different systems. The existing literature does not include the simulation or experimental setup of these systems together. However, the coupling of either of the two systems shows reliable results. For example, Table 17 shows various examples of direct integration of PV-PCM in building envelopes with significant results, supporting the technology; however, only a few studies show solar PV+PCM-TES integration where solar PV is used to charge the PCM_TES via electric heater for heating and the DHW of a house, which, in return, can decrease dependence on natural gas by up to 71% [275]. On the other hand, the integration of HP and PCM-TES can improve system performance, and the COP increased by 13.3% at an inlet water temperature of 30 °C [295]. Similarly, solar PV+HP is a recognized, efficient, and promising solution for electrifying thermal applications in buildings [346,347]. The integration of solar PV and HP can reduce building GHG emissions by up to 50% immediately and up to 90% over time [348,349].

There are, however, limitations to these systems. The inherent variability of solar flux and, thus, PV output, combined with the mismatch between generation and demand, increases power supply instability and drives the need for more ancillary services in Australia, which adds costs to the grid [350]. Additionally, rising peak demand from residential air-conditioning systems in the summer has become a significant challenge for both network operators and power generators [351]. It should be pointed out that AC use, aggravated by climate change, actually causes more grid challenges than PV in Australia [352]. When combined with heat pumps, a significant concern is the variable power exchange with the grid, which arises from the mismatch between the demand profiles of the heat pumps and the generation profiles of solar PV systems [332]. There are many solutions to these challenges such as demand response initiatives like load shaping, energy storage systems, dynamic voltage management, solar output management, dynamic time of use pricing, and restructuring solar energy remuneration [331]. A straightforward solution to enhance PV energy utilization and improve grid compatibility is the integration of TES with heat pumps in residential space heating systems [327,353]. Section 3.3 of this study shows the available literature on the integration of PV+HP with TES; however, these studies to date only include water tank-based TES. Although enough literature is available on PCM-based TES; however, their integration with PV+HP systems to supply residential thermal and electrical has not yet been explored. The PV+HP+TES system as a whole system gives significant results, as shown by Li et al. [331] who illustrate the potential advantages of integrating even a 5 kW rooftop solar PV system with an ASHP-TES water heating system. The results show that the yearly grid electricity demand can be reduced by 76% because of the use of the TES device [331]. This calls for a study on the PV+HP+PCM-based TES system as a whole sustainability integrated unit for the decarbonization of the residential sector and achieving carbon reduction goals.

There are also certain technical challenges when it comes to PV+HP+PCM-TES systems. The intermittent nature of solar PV demands a backup source; for example, the grid, a generator or combined heat and power. Although the mismatch between the solar PV generation and thermal load requirement can be solved by using PCM-TES (i.e., thermal battery) to store excess energy and use while required, the effectiveness of the TB will depend on the optimal system sizing, thermal response and PCM selection. In very cold climatic regions like northern Canada when the temperature declines below −25 degrees C, the system might require an auxiliary backup heater, which will increase the cost of the overall system. The charging of TB can be performed either using HP or using the external heating element which requires dynamic control strategies. Another challenge to TB is to extract the heat energy at the required flow rate and temperature, which can be solved by oversizing it and by using more sophisticated designs of TBs with multiple types of PCMs with different melting points. The experimental setup of the whole system can help find the solutions to all these challenges. In terms of achieving carbon neutrality, these setups can contribute to net zero energy houses by reducing dependence on the grid and making homes more resilient.

There is also a need for individual awareness and governmental policies to support these integrations and studies to make this system more available in the future. Despite being a proven technology, heat pumps still only make up a small portion of the worldwide heating market [181]. While ASHP offers an appealing option for electrifying the building sector, it often involves an investment. To encourage their adoption, many governments have implemented incentives aimed at promoting heat pump installation. Carroll et al. [354] argue that public policies are essential to make heat pumps financially viable. In rural China, the “coal-to-electricity” initiative aims to replace traditional heating sources like coal-fired boilers with ASHP [355]. In the United States, recent policies provide research funding and tax credits to promote the integration of electrified systems, including ASHPs, into grid-interactive efficient buildings [356]. Furthermore, Canada’s carbon pricing strategy imposes financial penalties on fossil fuel heating systems, fostering the transition to zero-emission electric technologies like heat pumps [357]. For example, the Canadian policies for HPs include the “Oil to Heat Pump Affordability Program” which helps eligible homeowners to receive up to USD 10,000 to switch from oil heating to heat pumps [358]. Despite these policies, the residential heat pump market share remains quite low at approximately 5% in Canada, with ASHPs dominating over GSHPs [359]. On the other hand, the HP still only meets 10% of the global heating demand in buildings [345]. This concludes that HPs need more flexible policy support to capture the heating market.

Similarly, within 1 TW global-installed solar capacity, 40% represents the distributed PV installation, out of which more than one-third represents residential sectors, meaning 130 GW of PV systems are deployed by households [360]. However, in Canada, distributed PV systems account for 1.3 GW_ac out of 4.3 GWac total installed capacity in 2022 [361]. In Canada, when it comes to the integration of residential PV systems with the grid, certain challenges are raised. The analysis report from Ontario Clean Air Alliance [362] points out the variation in grid application and connection fees from different utilities. For example, in London Hydro’s service areas, the fees begin at USD 1130 for solar projects under 20 kW but jump significantly to USD 28,250 for those exceeding 20 kW. In contrast, Alectra Utilities, which serves regions such as Barrie, Guelph, and Mississauga, maintains a standardized fee of USD 437 for connecting projects under 25 kW. To encourage the adoption of rooftop solar installations and promote larger systems among homeowners, it is equally important to focus on lowering the initial capital expenditure, as solar panel costs range between USD 8500 and USD 30,500, with the average 6 kW system priced around USD 12,700 [363]. It is important to note that these prices are before incentives and tax credits are applied. In addition, offering cash payments to homeowners rather than credits would provide a more immediate and tangible incentive. This calls for implementing new policies that establish fixed, low-cost grid connection fees for PV systems, which would also help alleviate financial barriers and support the widespread deployment of solar energy. There are more requirements of policies for supporting the PCM-based TES in the energy market to boost the deployment, cut their cost and their integration with the PV+HP system as, currently, TES is only responsible for ~1% of the global TES market [364]. Finally, PV+HP+PCM-based TES could become the simplest and most sustainable solution in the coming future for the decarbonization of residential sectors, but this would require policy support and public awareness along with experimental datasets and techno-economic analysis.

According to the International Panel on Climate Change, rooftop solar PV panels produced roughly 12 times less carbon footprint compared to natural gas and 20 times less than coal in terms of CO₂ emission per kWh of electricity generated [15]. Similarly, residential HP can reduce carbon dioxide emissions by 38–53% over a gas furnace [365]. On the other hand, the coupling of solar PV and HP is capable of reducing building GHG emissions by up to 50% immediately [348]. The coupling of solar PV+PCM-TES for a house can reduce the dependence on natural gas by up to 71% [275]. The result of solar PV+HP+TES integration shows that it can reduce the yearly grid electricity demand by 76% because of the use of the TES device [331]. These are promising methods to make major carbon emissions reductions to build upon for future research and deployment to reach policy goals for carbon emissions reductions [366].

5. Conclusions and Policy Implications

The current research focuses on the decarbonization of the residential sector, as it is the low-hanging fruit in achieving carbon reduction goals and it is a direct way to help citizens/consumers by reducing energy-related costs associated with fossil fuels. The results of this review show that substantial progress has been made in PV, HP and PCM-based TES, but this was generally individual progress or is in technological couples. Although many studies and evidence of carbon reduction are available for the individual system, i.e., solar PV system, HP system, TES, and PCM-TES along with their coupling, i.e., PV+HP, HP+TES, PV+TES, and HP+PCM-TES, the integration of all these three systems together as a whole unit has not yet been studied in detail. This calls for studies on the PV+HP+PCM-based TES system as a whole sustainability integrated unit for the decarbonization of the residential sector such that the system can supply both thermal and electrical loads of the house. One of the innovative approaches to coupling this system could be that the PV system would be used to supply the electrical loads of the house along with the electricity to run the HP to supply the thermal loads with the power grid as backup. The excess HP energy would charge the PCM-based TES, which would act as a storage unit to store the extra heat energy for later use, making the whole system sustainable and overcoming the limitation of individual sub-systems. Overall, there is still a lack of available literature and experimental datasets needed to provide triple integration of PV+HP+PCM-TES models for widespread implementation globally. These datasets could also be helpful in future studies to achieve the cost of energy and environmental impact of this integrated system. In addition, future work is needed for a full environmental life cycle analysis of such triple-integrated systems to provide appropriate policy guidance. Lastly, what is available technically for PV+HP+PCM-TES systems needs to be better shared to raise public awareness, and government policies should be considered in more detail to enable widespread implementation.