Special Issue “Advanced Phase Change Materials for Thermal Storage”

1. Introduction

Thermal energy storage using phase change materials (PCMs) is a research topic that has attracted much attention in recent decades. This is mainly because the potential use of PCMs as latent storage media not only covers renewable energy and building efficiency applications, but also the temperature control of electronic devices, batteries and even clothes. Although a number of companies worldwide are producing a variety of PCMs, advanced materials with improved properties and new latent storage concepts are required to better meet the specific requirements of different applications. Moreover, the development of common validation procedures for PCMs is an important issue that should be addressed in order to achieve commercial deployment and implementation of these kinds of materials in latent storage systems.

2. Advanced Phase Change Materials for Thermal Storage

The key subjects included in this special issue were related not only to materials in terms of new PCM formulations and concepts, validation and assessment procedures, characterization and simulation, but also to PCM applications in terms of implementation and testing in storage prototypes, innovative approaches and the simulation of novel storage modules for latent heat. Despite COVID-19 crises and lockdowns in most countries, there were still six papers submitted to this special issue, and five of them were accepted, which proves the quality of the research and the strong interest in the field of latent heat storage. In the following paragraphs, a summary of these papers with their most relevant contributions is presented.

The first paper included in this issue dealt with a procedure for selecting the appropriate PCM for two kinds of innovative compact energy storage systems implemented in residential buildings: the Mediterranean (MED) concept, intended for space cooling, and the continental (CON) concept, used for heating and domestic water [1]. The selection methodology consists of a qualitative decision matrix, which uses some common features of PCMs to assign an overall score to each material so that different options can be compared. The most important PCM features to be considered in the decision matrix for material selection are the melting enthalpy and temperature range, availability, cost and, in the case of the CON concept, the maximum working temperature range. Apart from the qualitative results, the authors made an experimental characterization of the best candidates, consisting of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), before making a final decision. This selection process led to various possible candidates, but two commercial PCMs were selected as the most promising ones: RT4 (T_melt = 4 °C) for the MED cooling system and RT64HC (T_melt = 64 °C) for the CON system providing house heating and domestic hot water.

The main contribution of this paper is that it provides a simple, quick tool for pre-screening a PCM before being implemented in any application and selecting at least the most promising candidates to be included in further validation tests.

The second paper presented a methodology that allowed comparing latent heat energy storage (LHES) modules with different designs with respect to their compactness and heat transfer performance [2]. Nowadays, many novel and promising heat exchanger designs and concepts have emerged, aiming to enhance the heat transfer inside LHES devices. However, the wide range of experimental conditions that can be found in the literature for their characterization makes it difficult to compare their performance. In light of this, the methodology described in the paper established just two key performance indicators (KPIs) which were minimally influenced by the experimental conditions: the compactness degree (ΦPCM) and a normalized heat transfer performance coefficient (NHTPC). In the paper, these KPIs were calculated for several LHES units already reported in the literature, allowing a leveled performance comparison with regard to operating conditions at different scales, while characteristics like geometry, structural materials and PCMs remained intrinsic. The robustness of the proposed KPIs was confirmed for units at different size scales by varying the heat transfer fluid mass flow rates and temperature levels. The evaluation procedure was applied to various LHES systems, and the most promising designs for different applications were identified and discussed. The authors clarified that the storage units analyzed were application-oriented and, in most cases, a high heat transfer rate was not a requirement, which led to low values for the KPIs. However optimized versions of the evaluated LHES systems are expected to deliver considerably higher performance indicators. Hence, the conclusions of this analysis should be considered as preliminary pictures of the general potential associated to each technological approach. The main contribution of this paper is that the methodology proposed is expected to open new paths in LHES research by allowing the leveled-ground comparison of technologies among different studies, facilitating the evaluation and selection of the most suitable design or designs for a specific storage application.

The third paper accepted in this special issue introduced the use of PCMs for the thermal management of lithium-ion batteries (LIBs), since temperature is an important factor affecting the working efficiency and service life of these devices [3]. In this work, the authors studied the thermal performance of two commercial batteries (Sony and Sanyo) under different working conditions: extreme conditions (inside a closed aluminum tube), natural convection cooling and PCM cooling with and without heat dissipation fins. The PCM used was a composite of wax with expanded graphite (T_melt = 52 °C), and the experimental results showed that the PCM was able to absorb some of the heat produced during both the charge and discharge processes and, hence, effectively reduce the temperature and keep the battery capacity stable. In fact, the tests performed at different discharge rates showed that the temperature decrease attained under PCM cooling was much higher for the Sanyo LIB (between 4.7 °C and 12.8 °C) than for the Sony LIB (between 1.1 °C and 2 °C), in both cases being compared with the natural convection experiments. The temperature reduction impact on the Sanyo LIB was stronger because this battery generated more heat due to its larger storage capacity. As for future developments, the authors suggested that further optimization of LIB thermal management in terms of surface temperature reduction could be achieved if a PCM with a higher latent heat was combined with heat dissipation fins. In my opinion, the most interesting contribution of this paper is that electrical storage and thermal storage working together can improve the performance efficiency of energy storage systems, which is one of the main challenges to be addressed and solved in future energetic scenarios.

The fourth paper resulted from the collaboration of several institutions and was developed within the framework of Annex 33/SHC Task 58 Material and Component Development for Compact Thermal Energy Storage, a joint working group of the Energy Storage (ES) and Solar Heating and Cooling (SHC) Technology Collaboration Programmes of the International Energy Agency (IEA) [4]. It consisted of a survey with a detailed description of the experimental devices present in those institutions and used for investigating the long-term stability and performance of PCMs under application conditions [5]. In fact, an important prerequisite to select a reliable material for thermal energy storage applications is to investigate its performance under real working conditions. In the case of solid–liquid PCMs, the long-term performance in terms of the melting and solidification processes should be ensured along the lifetime of the storage system, taking into account the conditions of the intended application. In this work, the different institutions presented up to 18 experimental set-ups and devices that allowed for performing thermal tests (cycling and constant temperature) not only for conventional PCMs, but also for the ones with stable supercooling, as well as phase change slurries (PCSs). Moreover, the paper introduced appropriate methods to investigate possible degradation mechanisms of PCMs. Considering the diversity of the devices and the wide range of experimental parameters, further work toward a standardization of PCM stability testing is strongly recommended. The main contribution of this paper is that it puts together many experimental facilities currently in use and the know-how of the corresponding institutions for assessing the long-term performance of PCMs, which certainly is a key issue for the commercial implementation of LHTS systems.

The last paper of this special issue presented a compact model of latent heat thermal storage (LHTS) for its integration in multi-energy systems [6]. In this way, the study developed a new modeling approach to quickly characterize the dynamic behavior of an LHTS unit. The thermal power released or absorbed by an LHTS module was expressed only as a function of the current and the initial state of charge. Moreover, the model also allowed for simulating even the partial charge and discharge processes. In general, the results were quite accurate when compared with a 2D finite volume model, with the advantage of the computational effort being much lower. Due to its simplicity, this model can be used to investigate optimal LHTS control strategies at the system level. In light of this, the authors implemented it in two relevant case studies: the reduction of the morning thermal power peak in district heating systems and the optimal energy supply schedule in multi-energy systems. However, this study describes the functioning of the LHTS unit at the system level only on the basis of numerical results. Hence, future work should also test the LHTS unit in a real case application to better quantify the model uncertainties.

The main contribution of this paper is the development of a simple model for LHTS modules that can be implemented in the simulation of multi-energy systems, although the model uncertainties still remain unquantified since it should be validated with data obtained from real applications.