Over the past decade, the research community has demonstrated increasing interest in advancing geothermal heating and cooling (GHC) systems through the integration of phase change materials (PCMs), tripling the number of scientific publications between 2021 and 2024. Within this context, this review evaluates two main application forms: PCM-based thermal energy storage (PCM-TES) units and PCM in ground heat exchangers (PCM-GHEs), the latter used with PCM-enhanced heat transfer fluids, PCM in heat transfer pipes, PCM in backfills, or PCMs deployed in ground vicinity. This review yields that on average, PCM integration improves heat transfer between the ground and the thermal load by 27 %, increases the coefficient of performance of heat pumps by 15 %, stabilizes heat transfer fluid temperature by 1–3 °C, and reduces GHE length by 10 - 90 %. For PCM-TES units, cost savings of up to 55 % are achieved with tariff-based operation. However, system performance is sensitive to both ground and phase change temperatures. Especially for PCM-GHEs, which require favorable operating conditions in both short- and long-term, and thermal conductivity enhancement of PCMs is often a necessity. Current technologies demonstrate a technology readiness level of 4–8, yet require validation through long-term, full-scale testing and comprehensive evaluations of cost effectiveness, as under-investigated aspects today.

This study explores the effectiveness of a high-temperature latent heat thermal energy storage (LTES) system incorporating Al-Si-based microencapsulated phase change material (MEPCM) composite pellets within a cylindrical packed bed. A parametric analysis was conducted to examine the impact of varying pellet sizes (1, 3, and 5 mm) and airflow rates (20–50 L min−1) on the efficiency of heat storage and discharge. The experimental approach included controlled charging and discharging cycles at temperatures ranging from 500 to 800 °C, with Reynolds numbers between 17.6 and 261. The findings indicate that the system achieved a maximum round-trip efficiency of 0.93, with no substantial gains observed beyond a Reynolds number of 150. Additionally, the results reveal the importance of minimizing heat loss to improve system efficiency, particularly during the discharge phase. These insights are crucial for optimizing the design and operational parameters of high-temperature LTES systems to enhance energy storage efficiency.

Reliable and cost-effective energy storage is essential to accelerate the adoption of renewable energy systems such as concentrated solar power (CSP) technologies. Single-tank Packed Bed Thermal Energy Storage (PBTES) offers a promising, lower-cost alternative to traditional two-tank systems for high-temperature storage. This study explores a hybrid sensible-latent PBTES system that integrates two types of Phase Change Materials (PCMs), strategically placed at opposite ends of a sensible-based PBTES, to enhance performance in terms of storage density and outlet fluid temperature stability. This is the first study to systematically evaluate metallic PCMs in multi-layered hybrid PBTES. A comprehensive numerical investigation, spanning PCM volume fractions from 0 to 30 % for each PCM, is conducted using a validated concentric dispersion model. The results show that PCM integration significantly boosts storage capacity, improves thermal stability, extends temperature plateaus during charging and discharging cycles and increases the energy density by up to 250 %. These hybrid configurations also extend the useful operation time by up to 220 % during charging and 300 % during discharging cycles with up to 250 % of useful energy capacity increase. Economic analysis showed a payback period of 4.8–5.5 years, with reductions in PCM layer at the top of the TES unit and encapsulation fabrication costs providing the most significant improvements in overall cost. While the hybrid system enhances temperature stability and energy utilization, it introduces trade-offs in terms of cost and efficiency, underscoring the importance of optimized PCM selection and its operating conditions. This work demonstrates the transformative potential of hybrid PBTES systems in delivering efficient, stable, and tailored energy storage solutions for future energy systems.

In this study, two novel metal-based microencapsulated phase change materials (PCMs) with melting temperatures of 577 °C (Al-Si) and 520 °C (Al-Cu-Si) are analysed for a next-generation concentrated solar power plant operating at 650 °C. Five storage units with different Al-Si to Al-Cu-Si volume ratios (1:0, 0:1, 1:1, 2:1) and height/diameter ratios (H/D ratios, 3:2, 2:3) are designed and benchmarked under the actual operating conditions, with the charge and discharge cut-off temperatures set for the return loop at 376 °C and 456 °C. The results show that the case with single Al-Cu-Si PCM leads to a higher average outlet temperature of 557.3 °C (8.7 °C higher outlet temperature), while the case with smaller H/D ratios shows higher energy storage capacity (on average 34 % higher capacity) due to smaller surface area of tank shell, resulting in less heat loss.

Thermal energy Storage has received widespread attention for its role in integrating variable solar energy. Latent heat thermal energy storage with phase change materials (PCMs) is regarded as a promising technology. In this study, two novel metal-based microencapsulated PCMs with melting temperatures of 577 °C and 520 °C are analysed for their application in the 650 °C next-generation solar power plant. Five storage units with different Al-Si to Al-Cu-Sivolume ratios (1:0, 0:1, 1:1, 2:1) and height/diameter ratios (3:2, 2:3) are designed and benchmarked under the actual operating conditions, with the charge and discharge cut-off temperatures set for the return loop at 376 °C and 456 °C.The results show that the case with single Al-Si PCM leads to higher average outlet temperature, which is good for steam generation, while the case with single Al-Cu-Si PCM shows higher storage capacity. The thermal performance of the cascaded models with two PCMs exhibit a compromise between high outlet temperature and high storage capacity. The height/diameter ratio of 3:2 shows the best performance considering outlet temperature and stored heat capacity. 

This chapter describes the development by Unite! (University Network for Innovation, Technology and Engineering), an alliance of seven European universities, of a new multiple Co-tutelle model for joint doctoral education. The chapter discusses what a Co-tutelle means, the Unite! model and how it was created, different student and faculty experiences, the new multiple Co-tutelle framework, and its individual annexes and foundational principles. Features that prohibit and promote this kind of multiple Co-tutelle model are reflected on, along with lessons learned.

This study proposes a life cycle assessment (LCA) framework to support the design of a building heating and cooling system with a ground source heat pump (GSHP) and phase change material thermal energy storage (PCM-TES) considering three key performance indicators (KPIs) in long-term performance, life cycle cost, and life cycle impact. It provides a method to map interlinkages of KPIs and identify key influence factors.   

This study reviews advancements in the use of phase change materials (PCMs) in underground thermal energy storage(UTES) systems. The primary application forms include vertical and horizontal ground heat exchangers (GHEs) and PCM underground storage units. The paper emphasizes the diverse potential of PCMs in UTES by selecting representative research that investigates various types of PCMs and their integration concepts. It highlights the role of PCMs in enhancing storage density and improving system efficiency.

Latent heat thermal energy storage (LHTES) implemented in residential heating systems has attracted attention for its role in peak/load shifting. A novel layout integrating LHTES with a heat pump is proposed to store low grade heat during off-peak demand period, later used as heat source for the heat pump during on-peak demand period. This novel layout is assessed according to different seasons, LHTES height-to-diameter (H/D) ratios, mass ratios of inflow water to radiator return water, and levelized cost of energy (LCOE). The results show that an overall increased amount of power input is required when utilizing LHTES, while it can shift 2.8–3.6 kW electricity from on-peak to off-peak. The case with an H/D ratio of 1.7 shows slight reductions in heating costs and LCOE as compared to a H/D ratio of 0.6. Considering heating costs, a mass ratio of 50 % performs better in December 2022 and a mass ratio of 10 % performs better in January 2023 due to different operating conditions. The heating costs of the integrated system are 1.0 %–2.1 % higher than those of the typical system due to limitations in the rated capacity of the heat pump and lower effectiveness of the shell-and-tube heat exchanger.

Many efforts are being made to mitigate the main disadvantage of most phase change materials - their low thermal conductivities - in order to deliver latent heat energy storage systems (LHESS) with adequate perfor-mance. However, the effect of applied methods is difficult to compare as they are mostly tested for different storage types and sizes and/or different boundary and initial conditions, which hinders rapid progress in the optimization of these approaches. In this work, a previously developed method for comparing the performance of LHESS is applied to experimental results of different storage systems under different conditions and subsequently analyzed and further refined. The main idea of the method is to normalize the power with the volume and a reference temperature difference and compare its mean value plotted over the normalized mean capacity flow of the heat transfer fluid (HTF). This enables the presentation of the results in a compact and easily comparative way. Attention has to be paid when it comes to the choice of the reference temperature difference, the reference volume and the method for calculating the mean value. Two variants of calculating the mean value (time-weighted and energy-weighted) and two variants of reference temperatures for determining the temperature difference to the inlet temperature of the HTF (initial temperature and melting temperature) are applied and discussed in detail. While the method significantly increases the comparability of results, none of the options listed above are without drawbacks. Approaches are shown to reduce or eliminate these drawbacks in the future. The recommendation for comparing different LHESS under different conditions is to use the method described here and clearly state the chosen reference temperature, reference volume and method for calculating the mean value.