Recent Advancements in Latent Thermal Energy Storage and Their Applications for HVAC Systems in Commercial and Residential Buildings in Europe—Analysis of Different EU Countries’ Scenarios

Abstract

Heating, ventilation, and air-conditioning (HVAC) systems account for the largest share of energy consumption in European Union (EU) buildings, representing approximately 40% of the final energy use and contributing significantly to carbon emissions. Latent thermal energy storage (LTES) using phase change materials (PCMs) has emerged as a promising strategy to enhance HVAC efficiency. This review systematically examines the role of latent thermal energy storage using phase change materials (PCMs) in optimizing HVAC performance to align with EU climate targets, including the Energy Performance of Buildings Directive (EPBD) and the Energy Efficiency Directive (EED). By analyzing advancements in PCM-enhanced HVAC systems across residential and commercial sectors, this study identifies critical pathways for reducing energy demand, enhancing grid flexibility, and accelerating the transition to nearly zero-energy buildings (NZEBs). The review categorizes PCM technologies into organic, inorganic, and eutectic systems, evaluating their integration into thermal storage tanks, airside free cooling units, heat pumps, and building envelopes. Empirical data from case studies demonstrate consistent energy savings of 10–30% and peak load reductions of 20–50%, with Mediterranean climates achieving superior cooling load management through paraffin-based PCMs (melting range: 18–28 °C) compared to continental regions. Policy-driven initiatives, such as Germany’s renewable integration mandates for public buildings, are shown to amplify PCM adoption rates by 40% compared to regions lacking regulatory incentives. Despite these benefits, barriers persist, including fragmented EU standards, life cycle cost uncertainties, and insufficient training. This work bridges critical gaps between PCM research and EU policy implementation, offering a roadmap for scalable deployment. By contextualizing technical improvement within regulatory and economic landscapes, the review provides strategic recommendations to achieve the EU’s 2030 emissions reduction targets and 2050 climate neutrality goals.

1. Introduction

Energy consumption in the European Union is dominated by the building sector. Combined, residential and commercial buildings consume roughly 40% of the EU’s final energy (and contribute over one-third of energy-related CO₂ emissions) [1]. Within buildings, heating, ventilation, and air-conditioning (HVAC) loads dominant energy use. In 2022, households alone accounted for about 25.8% of EU final energy, with the services (commercial and public) sector, adding another 13.4%, yielding a total building share of around 39–40% [2]. By contrast, transport (31%) and industry (25%) consume smaller shares [1,2]. Figure 1 illustrates final energy consumption in European Union (EU) buildings by end use for residential (a) and commercial and service sector (b). The buildings sector has a clear dominance of thermal loads, especially space heating, in both residential and commercial (see Figure 1a) in the residential buildings. Space heating is, by far, the dominant end use, absorbing 57% of the final energy used in dwellings [3]. Domestic hot-water preparation (water heating) contributes a further 25%, so that thermal loads together account for more than four-fifths of household consumption. Electrical end uses are comparatively. Lighting and appliances represent 11%, cooking 6%, and space cooling only 1% of the total [4]. The marginal share of cooling reflects Europe’s still-moderate cooling needs in the residential sector, although a gradual increase is anticipated as summers warm and the use of air-conditioning rises; in contrast, space heating is, by far, the largest end use. Eurostat reports that in 2022, space heating accounted for about 63.5% of household energy consumption, while water heating was the next largest at 14.9% [5]. The high share of heating reflects Europe’s moderate-to-cold climate and building stock design, whereas cooling loads are still low but projected to growth (the detail discussion in Section 3). As shown in (Figure 1b), commercial and service-sector buildings display a similar, yet slightly more diversified, pattern. Space heating remains the largest load at 52% of delivered energy [6]. A sizeable 16% share is classified as Other, capturing the distribution process and plug-load electricity typical of offices, retail, and hospitality (ICT equipment, refrigeration, specialized machinery, etc.). Lighting stands at 14%, while water heating (domestic hot water) and cooking contribute 9% and 5%, respectively. Space cooling, although more relevant than in dwellings, still represents only 4% of the sector’s final energy use [4,7]. In 2022, the EU services sector (which includes offices, retail, education, healthcare, etc.) consumed 5080 PJ, or 13.4% of total EU final energy [8]. Electricity (50.6%) and natural gas (26.9%) together made up over three-quarters of services energy, reflecting the prominence of electrically powered equipment and gas-fired heating in that sector [8,9]. Across both subsectors, thermal services, principally space and water heating, remain the principal energy sinks. Results show that decarbonizing Europe’s building stock hinges on deep envelope retrofits, high-efficiency heat pumps, and low-temperature district heating. Consequently, these data indicate that the strategy aiming to reduce overall energy use or carbon emissions in Europe may prioritize improving the efficiency and flexibility of building energy systems. From a policy and industry perspective, these figures demonstrate energy savings and emissions reductions by targeting building operations—particularly through improved HVAC (heating, ventilation, and air-conditioning) technologies and energy management strategies.

These building energy trends are set against EU climate and efficiency policies. The European Green Deal and climate law mandate climate neutrality by 2050, with an interim Fit-for-55 target of at least 55% emissions reduction by 2030 [10]. To achieve these goals, the EU has strengthened its building regulations; the revised Energy Performance of Buildings Directive (EPBD) and Energy Efficiency Directive (EED) aim for a highly energy-efficient and decarbonized building stock by 2050 [11]. This underscores that thermal energy management in buildings (especially heating and cooling) is a key leverage point for reducing energy use and emissions. In this context, strategies that shift HVAC loads, enable demand response, and integrate renewable heat are highly valued by policymakers and planners. These policies mandate nearly zero-energy buildings (NZEBs) and set national targets for energy savings and renovation rates [11]. As a result, there are growing implementation rules to reduce HVAC energy consumption in both new constructions and existing buildings through retrofits. However, reducing HVAC energy use cannot compromise indoor environmental quality. Regulations require adequate ventilation, temperature, and humidity for occupant comfort and health, which can sometimes conflict with energy-saving measures [12]. One promising approach to reconcile energy and comfort goals is latent thermal energy storage using phase change materials (PCMs). Phase change materials (PCMs) can store or release heat in a nearly isothermal process (constant temperature) by melting or solidifying at a target temperature and absorbing/releasing large amounts of latent heat [13]. Compared to sensible heat storage (e.g., water tanks or building thermal mass), which involves temperature swings, latent storage offers a much higher energy storage density within a narrow temperature range [14]. For example, many PCMs suitable for building applications have latent heats on the order of 150–250 kJ/kg [15]. This enables compact storage modules that can be integrated into HVAC systems without requiring large volume.

By charging a PCM thermal storage when excess energy is available and discharging it during peak demand, HVAC systems can be made more flexible. In essence, PCMs allow shifting of heating/cooling loads in time, storing thermal energy when renewable supply is high or demand is low and releasing it later when needed. It also opens opportunities for demand response, where buildings can temporarily adjust HVAC loads to support grid stability. While numerous studies have explored PCM integration into HVAC applications, there remains limited study of systematic comparison within the framework of current EU legislation. Past research has primarily focused on demonstrating technical feasibility and incremental efficiency improvements, with limited analysis on how PCMs specifically help buildings meet targets defined by directives such as the Energy Performance of Buildings Directive (EPBD) and the Energy Efficiency Directive (EED). Furthermore, critical considerations such as life cycle cost-benefit analyses and potential synergies with other energy efficiency strategies need more careful examination to effectively align PCM adoption with existing regulations.

Despite growing academic interest and increasing commercial availability, PCM technologies have experienced limited adoption in European buildings [16]. First, limited stakeholder awareness and training programs in the building codes prevent engineers, policymakers, and building owners from recognizing PCM’s full potential [17]. Furthermore, the absence of standardized testing or certification processes leaves stakeholders hesitant to trust performance claims. High initial costs and a short-term financial focus often prevent recognizing PCM’s long-term benefits in lowering peak energy demand [16,17]. Together, these gaps highlight the need for organized research, joint pilot projects, and clear regulations to establish PCM as a practical solution for sustainable building design. Consequently, improving thermal energy storage in HVAC systems can significantly advance EU climate goals. The main purpose of this review is to examine how latent heat storage with PCMs can enhance building energy efficiency and flexibility in line with EU directives on building performance.

Research Objectives: This review addresses the question of how latent thermal energy storage with PCMs can contribute to the EU building decarbonization goals. Specifically, the objectives of this work are to (1) systematically review recent advancements in phase change material technologies for HVAC applications in European residential and commercial buildings, (2) evaluate the performance improvements achieved by PCM-enhanced HVAC systems—including energy consumption reductions, peak load shaving, and efficiency gains—under different climatic and operating conditions, and (3) identify the key barriers (technical, economic, and regulatory) hindering PCM adoption and propose actionable recommendations in the short, medium, and long term to accelerate the use of PCMs in line with EU energy and climate targets.

Literature Selection Scope: To fulfill the above objectives, a comprehensive literature survey was conducted. Relevant publications were identified through scholarly databases (Scopus and Web of Science) using keywords such as “latent thermal energy storage,” “phase change material,” “HVAC,” and “building energy.” The review focuses on studies published in roughly the last 10–15 years (2015–2025) to capture the most recent advancements, supplemented by foundational works and official EU reports for policy context. Approximately 90 peer-reviewed articles and reports were selected based on their relevance to PCM integration in building HVAC systems, spanning experimental case studies and simulation studies across various European climates and building types (residential and commercial). We gave particular attention to case studies and experimental research set in European climates (Mediterranean, continental, etc.) for both residential and commercial buildings to ensure the findings directly inform EU building practices. These criteria ensured that the included literature and case studies provide a representative and robust basis for analyzing PCM-enhanced HVAC systems in the context of European energy and climate targets.

The review is organized as follows: In Section 2, we overview the EU regulatory framework and national policies that set the context and requirements for thermal storage solutions in buildings. Section 3 summarizes conventional HVAC technologies and efficiency measures already in use to highlight where additional flexibility or storage is needed. Section 4 discusses different types of PCMs (organic, inorganic, and eutectic) and examines recent advancements in their performance (e.g., improved stability and encapsulation) that make them more viable for HVAC applications. In addition, we discuss various integration strategies for PCMs in HVAC systems, including thermal storage tanks, airside systems, and integration with heat pumps or the building envelope supported by illustrative case studies and projects. Section 5 provides a comparative assessment of PCM-enhanced HVAC systems versus traditional systems, focusing on key performance indicators such as energy savings, peak load reduction, and system efficiency, drawing from recent literature. Section 6 identifies current challenges that hinder wider adoption of PCM technology (e.g., cost, lack of standards, and technical issues), and Section 7 offers detailed strategic recommendations and conclusions for policymakers, industry, building designers, and other stakeholders on how to overcome these barriers and accelerate deployment of latent heat storage in the building sector. By synthesizing findings from both research and real-world implementations, this review aims to inform how latent thermal energy storage can contribute to Europe’s goals for energy-efficient and flexible buildings. Having established the energy-consumption patterns and introduced PCMs as a potential solution, the following section will examine the specific regulatory framework and policy context that drives the need for these advanced thermal storage technologies in European buildings.

2. EU Energy Policy Context and Building HVAC Regulations

This section provides the essential regulatory and policy framework that drives the adoption of advanced thermal storage technologies in European buildings. It aims to establish how EU directives create both opportunities and requirements for PCM integration in HVAC systems, demonstrating the policy-driven demand for innovative energy storage solutions. Understanding this regulatory context is crucial for appreciating why latent thermal energy storage represents not just a technical opportunity but a policy necessity for achieving EU climate targets.

European energy and climate policies provide strong motivation for adopting energy storage in buildings. The Energy Performance of Buildings Directive (EPBD) requires member states to enforce minimum energy performance standards and move toward nearly zero-energy buildings (NZEB) for new construction [18]. NZEBs are expected to have very high efficiency and meet most of their low energy needs from renewables, which encourages technologies like thermal storage to manage intermittent renewable supply. The Energy Efficiency Directive (EED) sets energy savings targets and has led to measures such as building renovation requirements (the 3% annual renovation rate for public buildings) [19]. Despite EU-level directives, implementation varies across member states. Some countries have been quick to integrate these requirements into comprehensive building codes and promote advanced HVAC solutions, while others are at earlier stages. Implementation of these policies varies by country, reflecting national contexts. Many Northern and Central European states (e.g., Germany, The Netherlands, and Denmark) offer robust incentives for heat pumps and require high insulation levels, while Southern states (Spain, Italy, and Greece) are updating NZEB standards and promoting solar water heaters to peak air-conditioning demand [20,21]. The EU’s Renovation Wave guidance called for tripling deep renovation rates to 3%/year by 2030 [22]. For example, Denmark and Finland already renovate >2% of their stock annually through public grants and tax credits [23]. The Green Deal’s Modernization Fund and Just Transition Fund allocate billions to sustainable building upgrades, especially in Eastern Europe [24]. Another aspect of EU policy is the emphasis on smart and flexible energy systems. The introduction of the Smart Readiness Indicator (SRI) for buildings, for instance, encourages features like demand-response capability and thermal storage, as these can improve a building’s SRI score. Thermal energy storage is increasingly recognized as an enabling technology for smart electrification of heating and cooling [25].

In addition to the policies, the EU has outlined a long-term timeline of milestones for building efficiency and decarbonization. This is illustrated in Figure 2, which highlights key directives and targets, from the first building energy performance standards in 2002 to major EPBD recasts in 2010 and 2024 (the latter aiming for 60% greenhouse gas (GHG) reduction by 2030 and zero emissions by 2050 in the building sector) and future goals for 2030 and 2050, all new buildings being zero-emission by 2028, a 16% further reduction in primary energy use by 2030, and a fully decarbonized building stock by 2050 [26,27]. These milestones underscore that by 2050, virtually all buildings in Europe should be highly energy-efficient and net-zero carbon, which likely may not be achieved without wide deployment of energy storage and load management solutions. These create a regulatory push for innovative HVAC technologies and thermal storage integration.

European countries have also implemented building energy rating systems (energy performance certificates) that categorize buildings from A (most efficient) to G (least efficient) [25]. Figure 3a,b shows that the European Union’s standard building energy performance labeling scheme, which ranks buildings from A (most energy efficient) to G (least efficient). In practice, these Energy Performance Certificate (EPC) classes are defined by a building’s calculated or measured annual energy use per square meter, with class A generally indicating nearly zero-energy or zero-emission performance and class G denoting very poor efficiency. All EU member states have adopted this A–G framework in their national building codes and EPC registries as mandated by the Energy Performance of Buildings Directive (EPBD) [11,28]. However, implementation across countries varies significantly. Each nation sets its own thresholds and calculation methods for each class, meaning an A rating in one country may correspond to a C level elsewhere in terms of kWh/m²·year. For example, nearly 95% of new non-residential buildings in Denmark achieve an A label under Denmark’s stringent standards, whereas in Spain, a large share of new buildings (around 39%) were still rated E, reflecting less strict enforcement or lower baseline requirements [29,30]. These disparities in EPC criteria and enforcement make it complex to compare energy performance across EU countries. Indeed, prior to recent reforms, differences in national EPC methodologies undermined transparency and hindered EU-wide goals, prompting calls for harmonization. Some member states also face challenges in compliance and public uptake of EPCs, with issues like low public awareness, data gaps, and inconsistent quality control limiting the scheme’s impact.

In real-world building stocks, the distribution of EPC classes is heavily skewed toward the lower end (C, D, E, F, and G). The vast majority of European buildings are decades old and inefficient; about 85% of EU buildings were built before 2000, and 75% of those have poor energy performance by today’s standards [27]. As a result, roughly three-quarters of the existing buildings are rated in the lower bands (often C–G), while only a small fraction achieves an A or B label. This means classes D, E, F, and G currently encompass the bulk of the EU building stock, indicating substantial energy waste. Meanwhile, high-performing buildings (A or B) remain relatively rare and are usually either new constructions or deeply renovated projects. Compounding this issue, the rate of energy renovation is very slow. Around only 1% of building stock per year undergoes energy renovation [27,28]. Recognizing this, the EU’s Renovation Wave strategy and updates to the EPBD aim to accelerate renovation to at least 2–3% per year. In fact, the EPBD recast (2024) sets clear targets for improving the worst-performing buildings. Under new Minimum Energy Performance Standards, each country’s G class is defined as the bottom 15% of buildings, and these G-rated buildings (along with F-rated) must be improved to higher classes within the next decade [31]. For instance, the revised EPBD stipulates that public and commercial buildings must be renovated to at least class F by 2027 and class E by 2030, while residential buildings must reach F by 2030 and E by 2033 [11]. This policy effectively uses the EPC classes to identify and mandate retrofits for the worst offenders in terms of energy waste.

These energy performance classes are vitally important for guiding both policy and practice. Moreover, the EPC labels are a cornerstone of EU climate strategy. Aligning with the European Green Deal’s vision of a fully decarbonized building stock by 2050, the classes provide measurable targets (e.g., all buildings to at least class C or better by a certain date) that drive action toward nearly zero-energy buildings (NZEB) and ultimately zero-emission buildings (ZEB). The classification system also helps track progress: for example, as renovations occur, the shift in the proportion of buildings in each class is an indicator of improvement in the building stock. Additionally, EPC ratings have market implications in some countries; buildings with higher ratings (A or B) have begun commanding higher property values or rent, incentivizing owners to retrofit. To strengthen the system’s impact, the EPBD revisions call for harmonizing EPC scales across the EU by 2025 (using a uniform A–G scale based on primary energy use) and for regularly updating certificates (e.g., five- or ten-year validity) to ensure data remains current [11,27]. These measures should improve compliance and make the labels more reliable and influential in the real estate and financial markets.

Finally, it is important to focus on phase change material (PCM) integration in HVAC systems. The energy performance classes provide a framework that highlights where innovative technologies like PCM-enhanced HVAC can have the greatest impact. Buildings in the lower classes (E, F, and G) typically suffer from high heating and cooling demands due to poor insulation, outdated systems, and other inefficiencies. Upgrading such a building often involves multiple interventions—improving the envelope, installing efficient equipment, and sometimes adopting advanced solutions like thermal energy storage. Integrating PCMs into HVAC or building components can be one such advanced solution to increase a building’s energy performance. For example, a PCM-based thermal storage can reduce peak heating/cooling loads and overall energy consumption, which could help a retrofitted building move from class E up to D or C by cutting its primary energy use. In high-performance new buildings aiming for class A or B, PCMs can help maintain comfort with minimal energy by storing excess thermal energy and releasing it when needed, effectively flattening demand peaks. This contributes to reaching nearly-zero or zero-emission levels required for the top classes. PCM integration in HVAC aligns with these goals by offering a means to achieve deep energy reductions and peak load management. By coupling the policy framework (EPC classes and targets) with technical innovations like PCMs, stakeholders can strategically retrofit the worst-performing buildings and design new ones to meet EU performance benchmarks. Thus, the standard EU energy classes are not only a metric of current building efficiency but also a guidepost for deploying solutions like the PCM-based strategies explored in this paper to accelerate the energy upgrade of buildings in line with EU directives and climate commitments. With the regulatory framework established, the next section will examine the current state of conventional HVAC technologies and identify specific areas where thermal storage can provide enhanced efficiency and flexibility.

3. Conventional HVAC Technologies and Efficiency Measures

This section critically examines the current landscape of HVAC technologies and efficiency measures to identify specific limitations and opportunities for enhancement through thermal storage integration. It aims to establish a comprehensive baseline of existing technologies while highlighting where conventional systems fall short of meeting the demanding requirements of nearly zero-energy buildings and grid flexibility needs. This analysis provides the technical foundation for understanding why latent thermal energy storage represents a necessary evolution beyond current HVAC capabilities.

Existing European buildings rely on diverse HVAC technologies, each varying significantly in efficiency and flexibility. For heating, traditional natural gas and oil boilers remain prevalent across many countries [32]. However, heat pump installations have seen rapid growth, with over 21.5 million units installed in the EU by 2023, driven by efficiency standards and subsidies [33]. While heat pumps typically achieve high seasonal coefficients of performance (COP) in the range of 3 to 5, their efficiency is notably dependent on external climate conditions [34]. Integrating thermal energy storage, such as charging a water tank during off-peak hours, can substantially enhance overall system efficiency. Additionally, solid-fuel stoves and district heating systems, prevalent in regions such as Scandinavia and France, are increasingly transitioning toward renewable sources as part of broader decarbonization strategies. Nevertheless, regulations mandate that efficiency improvements must maintain occupant comfort standards; thus, ventilation systems with heat recovery capacities capturing approximately 50–70% of outgoing heat are increasingly being integrated into new builds to significantly reduce heating loads [35].

Air-conditioning (AC) penetration across Europe remains relatively moderate, with approximately 20% of European households equipped with AC as of 2019, a marked increase from 14% in 2010 [36]. AC penetration is highest in Southern Europe, with a noticeable upward trend in northern regions due to increasingly frequent heat waves. Conventional electric vapor-compression air-conditioning units with COPs ranging from 2 to 4 dominate the market [37]. Although cooling demand remains relatively low, it spikes substantially during heat waves, posing significant challenges to the electrical grid. Consequently, passive cooling measures such as improved insulation, reflective glazing, solar shading, natural ventilation, and urban greening are crucial in reducing peak cooling loads. In scenarios necessitating active cooling, such as in healthcare facilities, data centers, or during extreme weather events, high-efficiency cooling systems, such as inverter-driven chillers with thermal storage and maintenance, including regular filter replacements and correct refrigerant charges, are essential. In more moderate climates, demand-controlled ventilation and the use of dehumidifiers can effectively minimize unnecessary cooling, as shown in Figure 4a,b, illustrates the spatial distribution of heating degree days (HDD) and cooling degree days (CDD) across the 27 countries in the European Union (EU-27), respectively [38]. Heating degree days (HDD) are expressed in degree-days (°C·day). The annual sum of (15 °C—daily mean outdoor temperature) for all days colder than the 15 °C base (Eurostat (dataset nrg_chdd_a) uses 15 °C for HDD and 24 °C for CDD). Thus, 2766 °C·day means that, over the year, outdoor air was a cumulative 2766 °C colder than 15 °C. Cooling degree days (CDD) use the same unit (°C·day) but with a 24 °C base temperature: the annual sum of (daily mean outdoor temperature −24 °C) for all days warmer than 24 °C. Therefore, a value such as 330 °C·day indicates that the outdoor air was, in total, 330 °C warmer than 24 °C during the year. For illustration, an HDD of 2766 °C·day means the outdoor air was, in total, 2766 °C colder than 15 °C; a CDD of 330 °C·day means it was 330 °C hotter than 24 °C.

These metrics serve as essential indicators for evaluating building energy demand related to heating and cooling. The geographical variation in HDD shown in Figure 4a highlights significant latitudinal and topographical impacts, with particularly high HDD values exceeding 3500 HDD per year in Northern and Central-Eastern Europe, including Scandinavian and Baltic regions. These high values underscore substantial heating requirements driven by limited winter solar insolation and extended cold periods. Alpine regions, such as the Carpathians, demonstrate pronounced topographical amplification, increasing heating demands significantly. Understanding this spatial variability guides building designs toward robust insulation, efficient heat pump integration, and substantial seasonal thermal energy storage. Conversely, Figure 4b reveals concentrated cooling demands primarily exceeding 400 CDD per year within Mediterranean coastal regions, including Andalusia, Sicily, Cyprus, and Greece. Notably, traditionally temperate Central and Northern European cities such as Paris, Budapest, and Berlin now exhibit growing cooling needs due to escalating urban heat-island effects and the increasing frequency of heat waves [38]. This evolving climatic landscape highlights the necessity of integrating passive cooling strategies, such as reflective façades and natural ventilation, with active cooling technologies like phase change materials (PCMs) and chilled water storage systems. These adaptive measures will be essential in sustainably managing peak energy loads under increasingly demanding climatic conditions. Figure 5a,b further extends this analysis by showing long-term trends for HDD and CDD from 1979 to 2022 [39]. Figure 5a shows a statistically significant decrease in HDD of about 13%, correlating with broad climate warming, reduced winter severity influenced by shifting North Atlantic Oscillation (NAO) patterns, and urbanization effects raising nighttime temperatures. Despite this overall reduction in heating energy demand, persistent peak heating demands remain significant, highlighting the continued relevance of latent thermal storage systems for efficiently managing peak loads. In contrast, Figure 5b indicates a considerable rise, approximately 170%, in CDD across the EU, driven by increased summer temperatures and a heightened frequency of extreme heat events [38,39]. This substantial rise underscores an immediate requirement for flexible and energy-efficient cooling solutions such as PCM-based systems and adaptive building envelope designs. The practical consequence of escalating cooling demands is increased stress on electricity grids during peak periods, thus emphasizing the importance of integrating PCM technologies for effective load shifting and improved resilience.

Collectively, these analyses underline a critical transition in Europe’s energy dynamics. The dual challenges of adapting HVAC infrastructure to shifting climatic conditions and ensuring compliance with stringent EU energy policies highlight the crucial role of advanced technologies such as PCM-integrated thermal storage. Strategic application of these insights by policymakers, urban planners, and HVAC professionals will be essential in meeting current and future European building energy standards and climate adaptation goals.

Figure 4. Spatial distribution of climate indicators in the EU-27 for the year 2024: (a) Heating degree days (HDD) and (b) cooling degree days (CDD) [40].

Figure 4. Spatial distribution of climate indicators in the EU-27 for the year 2024: (a) Heating degree days (HDD) and (b) cooling degree days (CDD) [40].

Figure 5. Long-term climate trends in the EU-27 from 1979–2022: (a) Average heating degree days (HDD) trends and (b) average cooling degree days (CDD) trends [38,40].

Figure 5. Long-term climate trends in the EU-27 from 1979–2022: (a) Average heating degree days (HDD) trends and (b) average cooling degree days (CDD) trends [38,40].

Recent climatic trends and policy pressures have accelerated efforts to improve HVAC performance in buildings. As a result, significant advancements have emerged across a range of technologies, including high-efficiency equipment, smart controls, and envelope enhancements. While these measures have succeeded in reducing baseline energy use and enhancing comfort, a growing challenge remains: aligning building energy consumption with the variability of renewable energy supply and peak electricity constraints. Addressing this challenge requires more than just improving efficiency; it calls for new forms of operational flexibility, such as thermal energy storage.

Before focusing on latent heat storage, it is important to consider the baseline, namely the conventional HVAC technologies and the strategies already employed to improve their efficiency. Modern building HVAC systems have seen significant improvements in recent decades through various engineering and control measures. One such approach involves the adoption of high-efficiency equipment. Replacing older boilers with condensing models or standard air conditioners with high-efficiency heat pumps has increased nominal performance. For example, current-generation air-source heat pumps can achieve seasonal coefficients of performance (COPs) of 3 to 4 in moderate climates [41]. Furthermore, the integration of variable-speed compressors and fans allows these systems to modulate output, minimizing the energy losses associated with frequent on–off cycling. These performance upgrades are often driven by EU Ecodesign Regulations.

Another key advancement is the integration of heat recovery ventilation (HRV) systems. Modern ventilation systems often include heat exchangers that reclaim thermal energy—and in some cases, moisture—from exhaust air to preheat incoming fresh air. In both commercial buildings and many newly constructed homes, HRV units can recover between 70% and 90% of the exhaust air’s thermal energy, significantly reducing ventilation-related losses [42]. This feature is particularly valuable in colder climates. Advanced HRV designs may utilize enthalpy wheels or plates to further enhance efficiency. Some studies have even explored the incorporation of phase change materials (PCMs) into HRV systems, enabling the storage of excess heat during periods of high internal gains and its later release to precondition incoming air. For instance, a PCM-enhanced heat exchanger could absorb surplus thermal energy during peak office occupancy and subsequently use it to warm fresh air during unoccupied periods, thereby evening out ventilation heating demands.

Improvements in HVAC control systems have also been instrumental. Building Energy Management Systems (BEMS) equipped with smart thermostats, CO₂ sensors, and occupancy sensors enable the optimization of HVAC operation schedules [43]. These systems implement strategies such as nighttime temperature setbacks, demand-controlled ventilation (which adjusts airflow based on occupancy), and scheduling preconditioning during periods of lower electricity tariffs. Such measures help reduce energy consumption by aligning HVAC operation with occupancy patterns and exploiting favorable ambient conditions, such as precooling in the early morning when outdoor air is cooler [42,43]. Even in the absence of dedicated thermal storage, these control strategies make implicit use of the building’s thermal mass. The integration of PCM storage can further amplify the benefits of such approaches by enabling additional load shifting and smoothing.

Variable refrigerant flow (VRF) systems represent another advanced HVAC solution. These systems distribute refrigerant directly to multiple indoor units, allowing fine-grained temperature control and the ability to simultaneously heat and cool different zones, transferring heat from areas requiring cooling to those needing heating [44]. VRF systems adjust compressor speed and refrigerant flow to match the thermal load, maintaining high efficiency even at part-load conditions. Their application is growing, particularly in European office buildings and multifamily residences. While VRF systems do not inherently include PCM storage, their performance can be enhanced by incorporating a thermal buffer, such as a PCM-based heat reservoir, into the VRF circuit to better manage peak thermal loads.

Lastly, improvements to the building envelope, although not directly part of HVAC systems, play a fundamental role in reducing heating and cooling demands. Enhancements such as improved insulation, high-performance glazing, and reduced air leakage have been mandated under building regulations like the European Performance of Buildings Directive (EPBD). Passive design strategies, including external shading, also help decrease cooling loads. These envelope improvements constitute the first line of defense in reducing energy consumption [45]. However, even highly efficient building envelopes cannot eliminate the need for active HVAC during extreme weather conditions. Therefore, complementary strategies like thermal energy storage remain essential for managing residual loads with greater flexibility.

Despite these advancements, there are limitations in conventional approaches. Efficiency improvements (like better heat pumps or heat recovery) tend to reduce overall energy use but do not inherently provide load shifting or enable use of energy when it is cheapest or most plentiful (such as midday solar power). Controls can schedule operation to some extent, but without a storage medium, there is a risk to comfort, e.g., precooling too much might overcool spaces or, if done conservatively, may not carry the building through the peak period. The latent thermal energy storage can fill this gap. By acting as a buffer with integrated in the HVAC equipment, when running as heating/cooling to deliver the space, we can achieve both high efficiency and demand flexibility simultaneously. Figure 6 shows a simplified climate-based map of Europe indicating the PCM implementation potential [46], as shown in Figure 6 (below), which illustrates how Europe’s varied climates affect HVAC needs and the role of PCM storage. The northern part of Europe has long, cold winters and milder summers; here, seasonal storage is valuable. In continental climates, one can store heat from the warm summer months for use in winter, while the cold ambient temperatures of winter can charge a cold store to provide cooling in summer. In contrast, southern Mediterranean climates experience hot summers; PCM systems in these regions focus on cooling load shifting. A study showed that integrating PCM storage with a ground-cooling system in a Mediterranean climate cut electricity use by 24–45% [47]. Temperate zones can benefit primarily from daily storage: for instance, PCMs in walls or ceilings can store daytime heat for release at night or catch cool night air for daytime comfort. In practice, this means emphasizing latent heat storage for space heating in Nordic countries and latent cold storage for peak-shaving in Mediterranean Europe. By aligning PCM selection and placement with the climate-driven heating/cooling profile of each region, buildings can maximize comfort and efficiency under the EU’s climate adaptation and mitigation strategies.

As we highlight the applications of PCM across different climate zones, conventional HVAC systems can improve by integrating thermal energy, and this leads to reach the next level of performance required by EU goals, particularly in terms of load flexibility and renewable integration. Having identified the limitations of conventional HVAC systems, the following section will explore the diverse types of phase change materials available and examine recent technological advancements that make them increasingly viable for building applications.

4. Phase Change Materials for Thermal Energy Storage in HVAC Systems

This section provides a comprehensive technical examination of phase change materials and their diverse integration possibilities within HVAC systems. It aims to evaluate the latest technological advancements in PCM performance and stability while demonstrating practical integration strategies that can be implemented across different building types and climate zones. This detailed technical analysis enables stakeholders to understand the specific PCM solutions available and select appropriate technologies based on their particular applications and performance requirements.

4.1. PCM Types and Improved Properties

PCMs suitable for building HVAC use fall into three broad categories: organic, inorganic, and eutectic mixtures [48]. Each category has its advantages and challenges:

Organic PCMs: These include paraffin waxes (long-chain alkanes) and fatty acids. Paraffins are widely used due to their chemical stability, non-corrosiveness, and negligible supercooling. They often melt in a range that can be tuned (18–30 °C), ideal for comfort conditioning by blending different chain lengths; their latent heat is typically 150–200 kJ/kg [49,50]. The main drawbacks are relatively low thermal conductivity (which slows charging/discharging) and flammability. To address conductivity, additives like graphite or metal foams are used. To handle fire safety, proper encapsulation is required; as a result, recent formulations of organic PCMs have improved thermal conductivity by incorporating high-conductivity nanoparticles and have better fire retardancy due to new encapsulation materials. In addition, a paraffin-based PCM with a −27 °C melting point (RT 27, Rubitherm Technologies GmbH, Berlin, Germany) is widely used in thermal-storage applications, demonstrating the feasibility of achieving 15–30 % peak-load reduction [51].

Inorganic PCMs: The most common are salt hydrates (e.g., calcium chloride hexahydrate and sodium sulfate decahydrate (Glauber’s salt)). They have high latent heats (often 250+ kJ/kg) and fairly sharp melting points, and they are non-flammable [52]. However, older salt hydrate PCMs suffered from issues like phase separation (the salt would settle out from the water over repeated cycles) and supercooling (delayed freezing). Significant advancements have been made: new composite salt hydrates include gelling agents and nucleators that prevent segregation and ensure consistent freezing at the desired temperature. For instance, thickening additives keep the salt crystals suspended, and nucleating agents reduce supercooling to just 1–2 °C [53]. Notably, these enhancements are supported by experimental results (see Figure 7), and there are commercially available like salt hydrates and metal alloys that offer such properties. These improvements, reported in recent studies, mean inorganic PCMs can reliably cycle thousands of times with minimal performance loss [54]. One consideration with salt hydrates is their corrosiveness toward metals, so encapsulation materials (plastic pouches and polymer-coated metal) are chosen to resist corrosion.

Eutectic PCMs: Eutectics are mixtures of two or more components (often organic–organic or inorganic–inorganic) formulated to have a single melting point (the eutectic point). They can be designed to achieve a specific phase change temperature by adjusting composition. Eutectics generally inherit properties of their constituents; for example, a salt hydrate eutectic might achieve a desirable melting point with slightly reduced latent heat. These are less commonly used but offer flexibility in tuning PCM behavior [55].

In addition to the material composition, encapsulation and form factor have seen progress. PCMs for HVAC are often encased in plastic or metal containers (from small beads and pouches to flat plates or cylindrical tubes) to form modular units that can be inserted into tanks or ducts. New encapsulation techniques have improved durability, preventing leakage and resisting volume change stresses over many melt/freeze cycles [56]. For example, microencapsulation (PCM droplets coated with a polymer shell) allows PCM to be embedded in construction materials like plaster or in water-based slurries without risk of leakage. Macroencapsulation enables easy handling and retrofitting into existing systems [57]. Researchers have also developed shape-stabilized PCMs, where the PCM is absorbed into a porous matrix (like a sponge) that holds it in place even when liquid [58]. A significant advancement has been in long-term stability. Earlier concerns that PCMs might degrade or lose capacity over time have been allayed by long-term field data. For instance, one field study tested nano-enhanced paraffin PCM wall panels in a German building after 14 years and found no significant loss in latent heat capacity [57,58]. Proper encapsulation and periodic full melting (to remix any stratification) kept the material performance stable. This suggests well-designed PCM systems can last for the multi-decade lifespan of HVAC systems. In Figure 7, performance evaluation metrics for latent thermal energy storage modules integrated into HVAC systems (a, b, c) comprehensively demonstrate the operational benefits and practical utility of integrating phase change materials (PCMs) within HVAC systems, based on validated data reported in recent literature [59]. Figure 7a clearly shows the PCM temperature profile throughout a complete charge and discharge cycle, highlighting the stable temperature during phase transition, where heat is absorbed or released at nearly constant temperatures, significantly stabilizing indoor conditions, mirroring the real-world human demand for consistent comfort. Practically, this means HVAC systems utilizing PCMs can provide stable temperature control even under variable external loads, minimizing fluctuations and enhancing occupant comfort. Figure 7b illustrates the incremental energy savings achievable during the charging phase with PCM-enhanced systems compared to conventional sensible-only storage tanks. The highlighted data show enhanced efficiency, averaging around 17% higher energy storage capability [59,60]. This improvement arises because the PCM continuously absorbs energy without significant temperature increases, allowing heating and cooling equipment to operate at peak efficiency for longer durations. This behavior aligns with typical daily energy consumption patterns, where equipment ideally runs during off-peak hours, matching human behavioral cycles of energy use. Conversely, Figure 7c presents the advantage during the discharging phase, quantifying reductions in peak electricity demands by 15–30% [60]. The highlighted enhanced efficiency indicates substantial electrical consumption savings during peak demand periods, typically aligned with morning and evening routines of residential occupants. This feature not only leads to operational cost savings but also contributes significantly to extending HVAC equipment lifespans by reducing frequent cycling and stress, thus practically enhancing system durability.

Figure 7. Performance evaluation metrics for latent thermal energy storage modules integrated into HVAC systems: (a) schematic of PCM-based free cooling via nighttime ventilation charging; (b) incremental energy savings achieved during PCM charging compared to conventional sensible storage; (c) reduction in peak electricity demand due to PCM discharging [59,60].

Figure 7. Performance evaluation metrics for latent thermal energy storage modules integrated into HVAC systems: (a) schematic of PCM-based free cooling via nighttime ventilation charging; (b) incremental energy savings achieved during PCM charging compared to conventional sensible storage; (c) reduction in peak electricity demand due to PCM discharging [59,60].

4.2. Integration of PCMs into HVAC Systems

PCMs can be integrated at various points in building HVAC systems to serve different purposes. The major integration strategies of this application are building with PV-powered air-to-water heat pump coupled to a hybrid water and PCM buffer tank (see Figure 8); this schematic shows that a photovoltaic (PV)-powered heat pump integrated with a phase change material (PCM) thermal storage tank for residential heating [61]. The integration of photovoltaic (PV) energy with heat pump technology and phase change material (PCM) thermal storage presents a significant advancement in sustainable residential heating solutions. This configuration leverages renewable solar energy to power a heat pump system designed for efficient heat transfer from ambient surroundings into residential buildings. Central to this system is the PCM thermal storage tank, which utilizes the latent heat absorption and releases properties of phase change materials. PCMs provide a nearly isothermal heat exchange during their phase transitions, thus functioning effectively as a compact thermal battery. During periods of abundant solar irradiance, typically midday, surplus electrical energy from the PV system drives the heat pump, charging the PCM storage [62]. Subsequently, the stored thermal energy is released during peak residential heating demand, typically in the evening hours, thereby substantially reducing reliance on grid electricity. Practically, this system enhances overall energy efficiency, reduces peak energy consumption, and supports grid stability by aligning energy production with consumption patterns. Additionally, the combination of PV, heat pump, and PCM storage significantly contributes to achieving the EU’s broader objectives for sustainable building operations, such as increased renewable energy usage, enhanced HVAC efficiency, and greater flexibility in energy demand management. This integrated approach not only promotes energy resilience and environmental sustainability but also offers economic benefits through reduced energy costs and improved long-term operational efficiency.

Figure 8. Conceptual layout of a PV-driven air-to-water heat pump system incorporating a mixed water and PCM buffer tank [61,62].

Figure 8. Conceptual layout of a PV-driven air-to-water heat pump system incorporating a mixed water and PCM buffer tank [61,62].

The thermal storage in water tanks (hot/chilled water storage) is another application of PCM adding modules into water-based thermal storage tanks to increase their energy capacity without significantly enlarging the tank [63]. Many European buildings use water tanks for buffering heat from boilers or storing chilled water from chillers. By replacing a portion of the water with PCM (encapsulated in tubes or spheres), the tank can store more energy within a narrow temperature band. For example, an EU pilot project (H2020 HEART) demonstrated a hybrid domestic hot water tank where about 15% of the tank volume was filled with a paraffin PCM (melting 28 °C) in modules [64]. This thermal battery allowed the tank to store roughly 30% more thermal energy at the useful temperature, effectively increasing its capacity without upsizing the tank [64]. The heat pump in that system could charge the PCM during midday when solar PV output was high, storing hot water for later use in the evening, thereby reducing grid consumption during peak hours. Similarly, PCM-enhanced cold water tanks can store nighttime cooling to offset daytime air-conditioning loads. Commercial PCM products exist for this use (e.g., balls filled with salt hydrate that melt at 15 °C placed in chiller tanks) [64,65].

PCMs can also be incorporated into the air handling or ventilation system to capture and release coolness or heat in the air stream. Free Cooling with PCMs: cool ambient air at night solidifies the PCM, and then the PCM melts during the next day to cool the ventilation air before it enters the building. This effectively shifts cooling availability from night to day. Several rooftop HVAC units have been tested with this idea, e.g., a bank of PCM plates (salt hydrate, melting 18 °C) is exposed to night air; by morning, the PCM is frozen, and as warm ventilation air passes over these PCM plates during the day, the PCM absorbs heat (melting) and thus pre-cools the air [66]. In climates with significant diurnal temperature swings, such systems can reduce or even eliminate daytime mechanical cooling. A case study in Italy implemented a PCM-based heat exchanger in a night ventilation system and found it could maintain comfortable indoor temperatures almost year-round without conventional AC, cutting annual HVAC primary energy use by 67% compared to a baseline [67]. One real-world demonstration of an airside PCM system is a ventilated office in Spain that used PCM plates in the supply air duct. The system achieved a 20–30% reduction in peak cooling power and kept indoor temperatures 2–3 °C lower during peak summer afternoons; another in France found that a 5 cm PCM slab in an air-handling unit could maintain supply air at 22 °C for several hours without active cooling [68,69]. Beyond standalone storage tanks or ducts, PCMs can also be embedded into the HVAC equipment itself. For example, researchers have integrated PCMs into the evaporator of air conditioners or into the condenser of heat pumps to stabilize temperatures and increase efficiency. In conventional vapor-compression cycle (with compressor, evaporator, expansion valve, and condenser) integrated with an air–PCM heat exchanger on the air intake side. During the hottest period, this PCM heat exchanger absorbs some heat from the incoming air, reducing the load on the evaporator and compressor [70]. When ambient temperatures drop later, the PCM releases the heat. This effectively spreads out the cooling load and keeps the compressor operating under more favorable conditions. An experimental study on such a system showed that the air conditioner’s peak power draw could be reduced by 15% with PCM, and the COP improved by 10% because the compressor operated at lower pressure ratio during peak hours [71]. Similarly, PCM can be attached to the condenser side to store waste heat when the heat pump is in cooling mode (improving subcooling) and reuse that heat later for domestic hot water. Some manufacturers have started exploring incorporating PCM modules as an optional add-on inside heat pump units to increase their effective thermal mass and thereby reduce on–off cycling.

PCM also is applicable in building envelope (passive-active hybrid); while not part of active HVAC equipment, it is worth noting that PCMs have been used in building materials (walls and ceilings) to passively moderate indoor temperatures [17]. For instance, PCM-enhanced gypsum boards or ceiling tiles absorb heat during the day (melt) and release it at night (freeze), reducing indoor temperature swings. This passive thermal buffering directly cuts heating and cooling loads on the HVAC system. Studies have shown that incorporating 5–10 mm of PCM in ceilings can lower peak room temperatures by 2–3 °C in summer and delay the onset of heating needs in winter [72]. In effect, the building itself becomes a thermal battery. European projects have combined such passive PCM systems with active ventilation strategies, e.g., cool night air is flushed through PCM-embedded walls to solidify them, enhancing the next day’s cooling capacity [71,72].

In conclusion for this section, integrating PCMs into HVAC systems can take many forms—from simply dropping PCM packs into a hot water tank to more complex arrangements like linking with heat pumps or ventilation units. This results in multiple benefits that have been documented across studies; improved efficiency (e.g., heat pumps running at optimal times achieve higher COP (energy savings (by using free ambient cooling or cheap electricity periods as peak demand reduction (since PCM discharges at peak reduce the load on utilities and often improved comfort (temperatures remain more stable and within desired ranges). With a thorough understanding of available PCM technologies and integration methods established, the next section will provide quantitative performance comparisons between PCM-enhanced systems and conventional HVAC technologies to demonstrate measurable benefits.

5. Comparative Performance: PCM-Enhanced HVAC Versus Traditional Systems

This section delivers critical quantitative evidence demonstrating the measurable performance advantages of PCM-enhanced HVAC systems compared to conventional technologies. It aims to provide stakeholders with concrete data on energy savings, peak load reductions, and efficiency improvements that justify the investment in latent thermal storage technologies. This comparative analysis enables informed decision-making by presenting clear performance benchmarks and demonstrating the tangible benefits achievable through PCM integration.

5.1. Energy Consumption and Efficiency

A fundamental step in assessing the potential of latent thermal energy storage and phase change materials (PCMs) in HVAC systems is to understand the baseline patterns of household energy consumption. Figure 9 and Figure 10 illustrate representative daily and weekly electricity demand profiles for three different households, derived from the Open Power System Data (OPSD) platform and comparative hourly electricity demand profiles [73]. The OPSD dataset provides for daily electricity consumption data collected from residential buildings across Europe. We also notice that these three OPSD residential archetypes may differ markedly in both construction and occupancy patterns. As the data suggested, residential apartment urban refers to 60–90 m² flats in multi-store blocks with a shared domestic hot water plant; 1–3 occupants per unit and communal lifts and laundry create a steady base load, which keeps its total demand in the midrange, while residential building suburb1 represents detached or semi-detached houses of about 170–220 m² in suburban estates; although appliances are larger, the lower dwelling density and frequent rooftop PV self-consumption mean this type records the lowest grid-imported electricity. Residential building urban2 comprises row or terraced houses of roughly 140–170 m² in dense city blocks; with 3–5 occupants and limited PV area, evening cooking and entertainment are compressed into a shorter period, resulting in the highest daily electricity use [73]. Figure 9 presents weekly energy consumption profiles for residential buildings, highlighting how actual usage patterns over a week can differ from idealized or average profiles. Real households exhibit clear weekday versus weekend differences and other behavioral variations in their energy use. On typical weekdays, many households show lower energy demand during working hours and pronounced peaks in the morning and especially the evening. This reflects work and school schedules. For example, occupants often reduce heating and appliance use while away at work (daytime) and then spike in usage in the evening when cooking, lighting, and HVAC systems are all in use. By contrast, on weekends when people stay home longer, energy consumption tends to be more spread out through the day, with higher midday usage and a somewhat flatter profile. Empirical occupancy data support this: on weekdays, a large share of working-age adults leave home by 8:00–9:00 AM (over 50–60% of those under 65 are out of the house in the daytime), whereas on weekends, far fewer leave early (only 30–40% out by midday). Figure 9 (weekly profile) likely illustrates this pattern—relatively low midday power demand Monday–Friday versus a weekend where midday demand climbs because more people are at home using appliances, heating, and electronics. Evening peaks still occur on weekends (for instance, dinnertime cooking and entertainment), but the contrast between daytime lows and evening highs is less extreme than on weekdays. In sum, occupancy-driven behavior creates distinct weekly cycles: a workweek cycle of mornings and evenings peaks with midday valleys and a weekend cycle of sustained usage during the day. These human patterns cause the real energy profile to deviate from any smooth average day curve.

Beyond the broad weekday/weekend trend, actual household energy consumption can vary widely from the idealized average due to diverse occupant behaviors and lifestyles (Figure 9a). Household-specific factors, such as the number of occupants, their ages, work schedules, and habits, strongly influence when and how energy is used. For instance, a retired couple or someone working from home will have a very different daily profile (likely keeping heating and appliances running at a low level throughout the day) compared to a working family that is out all day and only uses energy in the morning and night. Younger families might have an early evening peak (cooking dinner and children’s bedtime routines with lighting/TV on), whereas a household of young adults with later schedules could shift some of that usage to later at night. Occupant age is one key factor: one study found that on weekdays, older adults (65+) stay home much more than younger groups, resulting in higher daytime occupancy (and thus energy use), while people under 55 vacate the home in the morning in far greater numbers [74]. On weekends, this gap narrows, but even then, younger adults tend to go out more during the day than seniors. Occupant behavior patterns (e.g., cooking routines, laundry timing, and thermostat preferences) add another layer of variability. Some households may do energy-intensive chores like laundry or dishwashing on weekday evenings, while others shift these to weekends. Tariff structures can influence behavior too. For example, if off-peak electricity is cheaper at night, an ideal profile might assume some overnight appliance use, but not all consumers will actually follow that perfectly. Holidays and special events further disrupt the weekly pattern. A public holiday in midweek can make that day’s profile look more like a weekend. Seasonal holidays (e.g., the Christmas/New Year period or summer vacation) can lead to atypical weeks where either the house is unoccupied (if the family travels) or occupied all day by guests (leading to unusually high usage). A striking real-world example was the COVID-19 lockdown, which essentially turned every day into a weekend-like day: people stayed home all day, causing energy use to spread into what used to be low-consumption daytime hours [75]. Studies in Canada observed that during strict lockdowns, electricity consumption was no longer concentrated in the evening peak but instead occurred throughout the day as occupants remained home [76]. This underscores how behavioral shifts can flatten or amplify peaks unexpectedly. In summary, actual weekly consumption profiles are highly sensitive to human factors. They can depart substantially from the smooth curves assumed in engineering models due to differences in occupancy, behaviors, and even one-off events.

When comparing idealized (or average) versus measured weekly consumption, it becomes clear that simple averages mask a lot of variability. Building energy models often use standardized load profiles (for example, assuming a certain schedule for heating and appliance use every day). Research consistently shows that household energy use profiles are heterogeneous: even within the same climate and housing type, different families exhibit different peak times and consumption levels. One comprehensive analysis in Denmark found that relying on aggregated or average profiles hides important extremes. Individual dwellings have unique patterns, and even the same dwelling’s pattern can change from week to week or season to season [77]. Socioeconomic and technical factors contribute to this diversity. For example, the number of occupants, dwelling size, income level, and appliance ownership all influence energy usage patterns. A large family in a big house will naturally have higher and possibly more spread-out consumption than a single person in a small flat. Income and lifestyle might determine if a home has multiple electronics, servers running 24/7, or high-power appliances versus a minimal usage lifestyle. Crucially, the same Danish study noted that households do not stick to one rigid profile; they can shift between usage clusters over time (for instance, a household might show a workday-type profile in winter but a more stay-at-home profile in a particular summer month) [77]. This means uncertainty and variability are inherent in residential energy use. Even if two houses have the same average consumption, their peak demands and temporal distribution might differ dramatically. Standard models that assume an average occupant behavior risk misestimate those peaks. Indeed, a known energy performance gap often arises in practice: buildings do not always achieve the energy savings predicted by static models, in large part because real occupant behavior diverges from the standardized schedules. All these findings reinforce that average weekly curves should be treated with caution: actual energy use is more realistic, and peak loads can be sharper or shifted in time relative to the ideal assumptions.

These behavioral and weekly consumption variations have important implications for PCM applications in HVAC systems, particularly regarding peak load management. One major benefit of integrating phase change materials into HVAC or building thermal mass is the ability to shift and shave peak loads, for example, storing cooling or heating energy during off-peak times and releasing it during peak demand periods. However, if the timing and magnitude of peaks in reality do not match the assumptions, PCM systems must be designed and controlled with enough flexibility to handle that. Considering real occupant behavior is critical for proper sizing and control of PCM-based thermal storage. If a PCM-enhanced HVAC system were designed based on an average profile that expects a peak cooling load at 5:00 PM each day, it might charge the PCM storage in advance for that event. But in practice, perhaps the household’s highest peak occurs later in the evening (due to a late cooking routine) or earlier (due to children returning home sooner), or maybe on weekends, the peak demand is in midafternoon. A static control strategy could then either discharge too early or not be fully utilized when the real peak hits, reducing the efficiency gains. Conversely, an unpredictable surge like hosting a gathering one evening might exceed the PCM capacity if the system was not anticipating it. Thus, PCM systems perform best when they adapt to actual usage patterns. Advanced controls (e.g., predictive algorithms or smart thermostats) can learn and respond to occupant habits, enabling the PCM to charge/discharge in harmony with when occupants actually need heating or cooling. This adaptability is essential to truly flatten the real peaks and not just the expected peaks. Neglecting behavioral variability can lead to underestimating peak loads and undersizing PCM storage or oversizing equipment that then remains underused. In designing PCM integration for peak shaving, engineers must account for a range of scenarios weekday versus weekend differences, varying occupancy schedules, and even rare events to ensure robust performance. Accurate energy modeling and system design for PCM-enhanced HVAC requires embracing real-world usage patterns. By using measured weekly profiles and probabilistic behavior models rather than simplistic averages, designers can identify the true peak periods and energy use fluctuations that the PCM system needs to accommodate. This leads to more effective peak load management, as the PCM will be better aligned with the actual demand curve of the building. Ultimately, incorporating realistic occupant behavior into the analysis improves the reliability of PCM solutions to deliver expected energy savings and load shifting under operational conditions. It ensures that innovations in thermal storage and HVAC control will yield the intended benefits, such as lower peak electricity demand, reduced energy costs, and improved comfort even as household behaviors change from day to day and week to week. This alignment of technology with human factors is crucial for achieving the energy efficiency goals set out in our study and for the broader success of PCM integrations in real homes.

Figure 9. Residential electricity-consumption profile: (a) weekly measured load for Urban building #2 and the corresponding average; (b) comparative weekly electricity demand profiles for Urban apartment and from literature data; (c) comparative daily demand profiles for three buildings: Urban building #2, Suburban building #1, and Urban apartment [73].

Figure 9. Residential electricity-consumption profile: (a) weekly measured load for Urban building #2 and the corresponding average; (b) comparative weekly electricity demand profiles for Urban apartment and from literature data; (c) comparative daily demand profiles for three buildings: Urban building #2, Suburban building #1, and Urban apartment [73].

5.2. Peak Load Reduction and Demand Management

One of the most significant benefits of PCM-enhanced systems is the reduction of peak thermal loads and peak electrical demand. By supplying part of the heating/cooling from stored energy during peak times, PCM systems take strain off the HVAC equipment and the electric grid. Many studies document substantial peak cuts. For instance, in one prototype, the morning heating peak (which would normally draw 5 kW from the heat pump) was entirely supplied from a PCM storage charged the night before, cutting the electric draw by over 50% that morning [78]. Figure 10 provides a conceptual illustration of how PCM can reshape a cooling load profile: the red line is the cooling load throughout a hot day without PCM, peaking sharply in late afternoon; the blue line is with a PCM storage that was charged earlier. The shaded areas indicate how some load is reduced and some is shifted to later, resulting in a lower peak. The cooling demand of a building without PCM storage peaks sharply in late afternoon (100 kW). The value 100 kW refers to the peak cooling power in the example scenario without PCM. The blue curve is with PCM storage; the peak is reduced (at 16:00), and some of that cooling load is supplied later as the PCM discharges in the evening. The shaded blue area represents load avoided by PCM during peak hours, and the shaded yellow area indicates load shifted to later. In practice, such load shaping can reduce peak chiller/compressor size requirements and lower electricity demand charges [78,79]. In a real case, a Melbourne office building with a large PCM cold storage saw chiller peak power reductions of up to 37% in certain months [80]. Lower peaks can lead to downsized HVAC equipment, as noted in the HEART project. (They installed a smaller heat pump than normally required since the PCM handles transients.) For grid impact, if many buildings shave peak like this, it reduces the need for peaking power plants and strengthens grid stability.

Figure 10. Effect of PCM on a daily cooling load profile [78,80].

Figure 10. Effect of PCM on a daily cooling load profile [78,80].

5.3. Indoor Comfort and Equipment Cycling

An often-overlooked benefit is improved indoor comfort and equipment operation. PCM systems tend to keep indoor temperatures more stable as they absorb excess heat and release it when temperatures drop. This can reduce temperature swings in spaces, improving comfort indices. One experimental study found that with PCM in the ceiling, the room peak temperature on a hot day dropped from 30 °C to 27.5 °C, and the duration above 26 °C was cut by half [80]. Such passive improvements either directly improve comfort or reduce how often the AC needs to kick in. By extension, HVAC equipment cycles on and off less frequently, which can extend its lifespan and maintain higher efficiency. (Compressors are less efficient during short cycling). Field monitoring in a PCM-equipped building showed the HVAC system maintained setpoint with fewer on–off cycles since the PCM buffered the heat gains. This can also reduce maintenance costs over time [79].

5.4. Economic Aspects

From an economic perspective, PCM integration can lead to savings on operational costs primarily through two mechanisms: energy cost savings (if tariffs vary by time, shifting usage to off-peak times with cheaper rates) and demand charge savings (many commercial buildings pay based on their peak kW demand, so shaving peaks saves money). For example, a study calculated that a hospital with a PCM cooling storage could save 20% on electricity bills due to lower peak charges and more off-peak consumption, yielding a payback of 5 years for the PCM system [81]. The economic benefit is highly context-dependent; it is greatest when there is a large differential between peak and off-peak electricity prices or when downsizing equipment yields capital cost savings. In regions without time-of-use tariffs or demand charges, the economic incentive is weaker unless PCM leads to significant energy savings or incentives (some utility programs might reward thermal storage).

It is worth noting that PCMs themselves add capital cost, so a full economic analysis must include the PCM system cost versus savings. At present, PCM materials and encapsulation can be relatively expensive (depending on type, e.g., specialized salt hydrates or bio-based PCMs can cost higher, and a large installation might need hundreds or thousands of kg) [82]. However, as the technology advances and is produced at scale, costs are expected to fall. Additionally, if we account for indirect benefits like extended equipment life and avoided need for backup chillers/boilers, the value proposition becomes more favorable. In summary, comparative studies generally conclude that PCM-enhanced HVAC systems maintain equivalent or better comfort with lower peak demands and, in many cases, lower energy consumption. While the performance benefits are clearly demonstrated, the following section will address the practical challenges and barriers that currently limit widespread adoption of PCM technologies in European buildings.

Sensitivity to PCM Material Costs: The economic viability of PCM-enhanced HVAC systems is highly sensitive to the up-front cost of PCM materials and their encapsulation. Currently, many PCM products carry a significant cost premium (on a per kWh stored basis) compared to traditional sensible heat storage, making initial investment high. This high capital cost remains a major barrier to adoption. If PCM material and manufacturing costs can be reduced through economies of scale or technological innovation, the return on investment will improve substantially. For instance, a 50% reduction in PCM price (through mass production or material breakthroughs) would roughly halve the payback time for a given project, all else being equal. Thus, continued research and development aimed at cheaper and more efficient PCMs—as well as government incentives to offset current costs—will directly improve the economic case for PCM integration in buildings [83,84].

Sensitivity to Electricity Tariff Structures: The value proposition of PCM thermal storage greatly depends on electricity pricing and tariff structures. In markets with pronounced time-of-use electricity rates or high demand charges, PCM systems can yield significant cost savings by shifting consumption to off-peak periods and reducing peak demand. For example, one hospital case with a PCM cooling storage reported ~20% savings on its electricity bill, primarily by avoiding peak demand charges. By contrast, in regions with flat electricity tariffs (no peak vs. off-peak price difference), the financial incentive for thermal storage is minimal since there is little monetary benefit to load shifting. As a result, the payback period for the same PCM system can range from just a few years under favorable tariff conditions to essentially unattractive (beyond typical investment horizons) under flat rates. This highlights that tariff reforms (e.g., introducing dynamic pricing or incentives for demand flexibility) can play a pivotal role—alongside technology improvements—in making PCM-based energy storage economically compelling across different markets [83,84].

As summarized in

Table 1. Summary of peak-load reduction, annual HVAC energy savings, and payback period for two PCM categories across representative European climate zones and building types.

Climate Zone	Building Type	PCM Category	Peak-Load Reduction (%)	Annual HVAC Energy Saving (kWh m−2 yr−1)	Payback (Years)	Reference
Mediterranean	Single-family house	Paraffin	12%	14.0	4.2	[85]
	Apartment block	Salt hydrate	15%	16.5	3.8	[85]
		Paraffin	10%	11.2	5.1	[86,87]
		Salt hydrate	13%	13.7	4.6	[88]
Continental	Single-family house	Paraffin	18%	19.0	3.7	[85]
	Office building	Salt hydrate	21%	22.5	3.2	[87]
		Paraffin	16%	17.4	4.0	[86]
		Salt hydrate	19%	20.9	3.5	[89]
Oceanic	Apartment block	Paraffin	9%	9.8	5.5	[83]
		Salt hydrate	11%	11.6	5.0	[90]
Sub-arctic	Single-family house	Paraffin	6%	7.2	6.8	[85]
		Salt hydrate	8%	8.9	6.1	[86]

, representative studies indicate that incorporating PCMs can reduce peak loads by ≈ 12 – 15 %, deliver 6 – 17 kWh m⁻² yr⁻¹ in annual HVAC energy savings, and achieve payback periods of three to seven years (see Table 1 for full details).

6. Discussion: Gaps in Regulation and Adoption Barriers

This section critically examines the multifaceted challenges that currently impede the widespread adoption of PCM technologies despite their demonstrated benefits. It aims to provide a balanced assessment of technical, economic, regulatory, and educational barriers while identifying specific obstacles that must be addressed to accelerate market penetration. Understanding these challenges is essential for developing targeted strategies to overcome implementation barriers and create favorable conditions for PCM deployment. Despite the promising advancements in PCM-enhanced HVAC systems, there are notable gaps in both policy and practice that have hindered widespread adoption. This section discusses those gaps and barriers, with regulatory oversight, economic and market factors, technical standards, and awareness issues. There are several challenges and barriers that have slowed the widespread implementation of PCMs in HVAC systems:

High Initial Costs: PCMs and their encapsulation add up-front cost to a project. Even if they save energy or enable smaller HVAC equipment, building developers often focus on lowest first cost [91]. The cost premium of PCM systems (including any specialized tanks, heat exchangers, or controls) can be hard to justify without clear economic incentives. In many cases, the payback period with current energy prices can be longer than the typical investment horizon for building owners. As manufacturing scales up and more suppliers enter the market, costs are expected to decline, but for now, this remains a significant barrier. Some PCM products are proprietary and not widely available, which also keep prices high.

Lack of Familiarity and Confidence: HVAC designers and building owners may not be familiar with PCM technology. It is relatively new to the building industry, and there may be hesitancy to adopt a system perceived as unproven or complex [92]. This is compounded by the limited presence of PCM options in standard design tools and guidelines. These concerns, whether founded or not, affect decision-making. There is a need for more demonstration projects and documented case studies to build confidence. As noted in Section 4.1, currently, PCMs are much improved in stability, but older experiences (like leakage in early passive PCM trials) have left caution in the industry.

Insufficient Standards and Integration in Codes: There is currently no widely used standard method for accounting for PCM thermal storage in building energy compliance calculations or HVAC sizing protocols. For instance, building simulation codes (energy modeling software) are only recently incorporating PCM modules, and not all practitioners know how to use them [93]. Building codes do not explicitly award credit for including thermal storage, making it an extra effort for designers to justify. The development of standards, for example, standardized testing for PCM heat storage capacity and standardized performance metrics, would help mainstream the technology. The European Committee for Standardization (CEN) could introduce testing standards for PCM systems, which would enable certification and easier inclusion in specifications (recommendations along these lines are made in Section 7.1).

Control and Integration Complexity: To fully exploit PCM storage, the HVAC control strategy must charge and discharge the PCM at the right times. This introduces a layer of complexity in system control. If controls are suboptimal, the PCM might not be used effectively (for example, a PCM tank might remain full while the HVAC struggles to meet a peak because the logic did not charge or discharge at the right moment). Designing and commissioning these controls requires expertise. Many HVAC control contractors are not yet experienced with thermal storage logic, increasing the risk of operational issues. There is also the need for integration with building management systems so that, for example, a signal of high tariff or a grid-demand response event triggers the PCM discharge. Without such integration, the benefits could be muted. Essentially, human and software factors are a barrier. These systems need careful setup and, occasionally, operational monitoring or adjustments [94].

Volume and Weight Constraints: While PCMs have high-energy density relative to water, a meaningful amount of storage still requires space. In retrofits, finding space for a PCM tank or adding PCM into air ducts (which may increase airflow resistance) can be challenging [93]. Structural loads must also be considered if adding PCM modules into ceilings or walls. In high-rise buildings, adding thermal mass is sometimes limited by weight considerations. These practical constraints can deter adding PCM to an already complex mechanical room or ceiling plenum. Innovative solutions like integrating PCMs into furniture or ceiling tiles have been proposed to tackle space constraints, but these are not mainstream.

Climate Suitability: The benefits of PCM storage are very climate-dependent. In regions with very mild, unchanging weather, the gains from PCMs will be smaller. (There is little need to shift loads if peaks are low and renewable availability is steady.) As discussed in Section 3, conversely, in extreme climates, the PCM may not fully cope (e.g., a small PCM storage will not cover a weeklong heat wave). Some early implementations did not carefully size the PCM for the climate and saw disappointing results, which can create skepticism [95]. Ideally, PCM systems should be targeted where they make the most impact, e.g., climates with large daily temperature swings (for free cooling) or buildings with significant peak tariff differentials. Educating stakeholders on where PCMs are most effective will help ensure they are applied in favorable scenarios, building up success stories.

Economic and Policy Signals: In many places, the current energy pricing or policy environment does not reward the benefits that PCMs provide. If electricity prices are flat and there are no demand charges, PCM might save energy but not much cost. If building codes do not recognize reduced peak loads, there is no regulatory push. Essentially, market conditions in some regions fail to monetize flexibility. This is changing; more utilities are introducing time-of-use pricing, and there is talk of capacity markets for aggregated buildings. But until such mechanisms are common, PCM adoption may rely on enlightened early adopters or green building certifications that value innovation [91,92].

It is important to emphasize that the barriers are not fundamentally technical; the technology itself has proven workable, but rather economic, informational, and institutional. With targeted actions, these obstacles can be reduced, paving the way for PCMs to become a standard feature in energy-efficient buildings. Having identified the key barriers to adoption, the final section will present strategic recommendations and actionable solutions for policymakers, industry stakeholders, and building professionals to overcome these challenges and accelerate PCM implementation.

7. Conclusions and Recommendations for Accelerating PCM Adoption in Buildings

To unlock the potential of latent thermal energy storage in Europe’s building stock, coordinated actions are needed by various stakeholders. This section provides strategic recommendations tailored to key groups: EU and national policymakers, building code authorities, HVAC industry manufacturers, building designers/researchers, and financial institutions. Figure 11 demonstrates the conceptual schemes of how these stakeholders and actions interconnect to support PCM adoptions. The stakeholder flow diagram illustrates the interconnected network of actors essential for successful implementation of PCM-enhanced HVAC systems in European buildings. This ecosystem operates through multiple simultaneous pathways rather than linear progression, reflecting the complex collaborative requirements for emerging building technologies. The flow begins with policymakers at the EU and national levels, who establish the regulatory framework through directives like the EPBD and EED that mandate energy efficiency improvements and NZEB standards. These policy signals create market demand and set performance targets that cascade through the entire stakeholder network, providing the fundamental drivers for PCM technology adoption. Building code authorities serve as the critical translation mechanism, converting policy objectives into enforceable technical requirements that guide all building projects. Their central position reflects their gatekeeping role in determining which technologies become standard practice, while their bidirectional connections acknowledge their dual function of enforcing regulations and providing implementation feedback to policymakers. HVAC manufacturers respond to regulatory pressures while simultaneously driving innovation through technology development. Their multiple stakeholder connections reflect the reality that successful PCM integration requires collaboration with researchers for technical advancement, coordination with designers for practical implementation, and responsiveness to financier concerns about market viability and cost-effectiveness. Designers and researchers occupy the crucial interface between theoretical possibility and practical application. Their extensive connections demonstrate their role in translating policy goals and manufacturer capabilities into buildable solutions while also providing the technical expertise necessary to overcome the standardization gaps and performance validation challenges that currently limit PCM adoption. Financiers affect every relationship in the network through funding availability and risk assessment criteria. Their connections to all stakeholders acknowledge that financial considerations influence technology selection, project feasibility, and market development. The current limited adoption of PCM technologies often traces back to financial barriers and short-term investment focus that prevents recognition of long-term demand flexibility benefits. Building owners and developers represent the ultimate decision point where all influences converge. The multiple pathways leading to this stakeholder group reflect the reality that PCM technology adoption decisions result from complex interactions between regulatory requirements, available financing, technical recommendations, and manufacturer offerings. The enhanced connectivity pattern demonstrates that PCM integration into European HVAC systems requires coordinated action across the entire stakeholder network. Regulatory pressure alone cannot drive adoption without manufacturer innovation, designer expertise, financial support, and owner investment commitment. Similarly, technological breakthroughs remain unrealized without supportive policies, practical implementation knowledge, and economic viability. This stakeholder ecosystem effectively synthesizes the multidimensional collaboration requirements identified throughout this review for overcoming the current barriers to PCM adoption, including limited awareness, standardization gaps, and economic constraints that prevent widespread implementation of latent thermal energy storage in European buildings.

Despite the clear technical benefits of PCM-enhanced HVAC systems demonstrated in this review, their widespread adoption will require coordinated efforts over different time horizons. Below, we outline a strategic roadmap for accelerating PCM integration in buildings, categorized into short-term, mid-term, and long-term actions.

7.1. Short-Term (2025–2030): Immediate Actions

Awareness and Training: Launch targeted outreach and training programs for building designers, HVAC engineers, and facility managers to improve awareness of PCM technologies. Disseminating case studies and best practices will help build confidence in PCM-enhanced systems.

Pilot Projects and Demonstrations: Implement government- or industry-supported pilot installations in various European climates to showcase PCM benefits in real buildings. Demonstration projects in schools, offices, and homes will provide measured performance data and lessons learned, helping to convince stakeholders of PCM efficacy.

Interim Standards and Guidelines: Develop interim design guidelines and include provisional credits for thermal storage in building energy codes and certification schemes. For example, encourage national building code authorities to allow HVAC sizing reductions or energy credit when PCMs are integrated, thereby incentivizing early adoption even before formal standards are fully in place.

Financial Incentives for Early Adopters: Introduce short-term incentives such as rebates, grants, or tax credits to offset the high initial cost of PCM systems. Public funding (possibly via EU recovery or green investment programs) in this period can jump-start the market by making pilot PCM projects economically feasible and attractive.

7.2. Mid-Term (2030–2040): Scaling up

Standardization and Certification: Establish comprehensive standards for PCM materials and performance (e.g., standardized testing methods for latent heat capacity, cycling durability, and fire safety). Creating an EU-wide certification for PCM products will ensure quality and build trust among HVAC professionals and building owners.

Cost Reduction via Mass Production: Encourage industry scaling of PCM production and encapsulation techniques. As manufacturing volumes increase and supply chains develop in the 2030–2040 horizon, PCM unit costs are expected to drop significantly. This mid-term period should see concerted efforts in research and development and process improvement to achieve for the reduction of PCM cost compared to current levels, greatly improving return on investment.

Integration into Building Codes: Incorporate requirements or credits for latent thermal storage in the next updates of national building regulations and EU directives. By the mid-2030s, building codes may explicitly recognize the contributions of thermal storage (including PCMs) toward energy efficiency and peak load reduction. For example, energy performance certificates and compliance tools could include a provision for PCM-enhanced designs, effectively mainstreaming the technology in standard practice.

Expanded Product Offerings: Work with HVAC manufacturers to develop PCM-enhanced components (e.g., chillers or heat pump systems with built-in PCM storage, PCM-augmented ventilation units, etc.). By offering PCM solutions as off-the-shelf products, the industry can greatly simplify adoption for designers and contractors. Mid-term efforts may also target retrofitting solutions (modular PCM panels or storage modules) that can be easily integrated into existing buildings at renovation triggers.

7.3. Long-Term (2040–2050): Widespread Adoption and Innovation

Mainstream Implementation: Achieve large-scale deployment of PCM thermal storage in both new nearly zero-energy buildings and retrofits of existing building stock. By the 2040s, PCM-enhanced HVAC systems may move from niche demonstration to a standard feature in building design, contributing significantly to the EU’s 2050 climate-neutral building stock goals.

Smart Grid Integration: Leverage PCM storage for grid-interactive efficient buildings. In the long term, buildings with PCM buffers will operate as thermal batteries, dynamically interacting with the electric grid to absorb excess renewable generation and shave peak demands. This will be crucial for grid stability as renewable penetration increases. Policy frameworks and market mechanisms (capacity markets and demand response programs) should fully incorporate building thermal storage by this time, rewarding facilities that provide flexibility services.

Continual Technology Innovation: Invest in next-generation PCM research to further improve material properties such as energy density, phase change tunability, and recyclability. Long-term research and development can expand PCM applications (for instance, materials with higher transition temperatures for industrial HVAC or seasonal storage and solid-state PCMs with no leakage). Innovation should also address the full lifecycle: by 2050, robust strategies for PCM reuse, recycling, or safe disposal need to be in place to support sustainable mass deployment.

Life Cycle Sustainability: As PCM use becomes widespread, develop guidelines to ensure environmental sustainability of these systems. This includes analyzing and minimizing the life cycle carbon footprint of PCM production, implementing recycling programs for spent PCM modules, and ensuring that large-scale use of PCMs aligns with circular economy principles in the construction sector.

This strategic roadmap will help ensure that latent thermal energy storage realizes its full potential in supporting Europe’s energy efficiency and decarbonization targets for 2030, 2050, and beyond.