With global population growth and our modern lifestyle, our energy demand keeps increasing. Our ever-increasing demand for energy should be met by renewable energy for a sustainable future. Both sustainability and environmental concern contributed to the growth of renewable energy at the international level, while most of the countries set national goals and made significant progress. In Australia, for example, 27.7% of the generated electricity was from renewable sources in 2020 with 50% annual increase in solar generation [1]. Tasmania, the Australian island state with just over 10 TWh annual energy demand, is currently running on 100% renewable energy. Heavily relying on wind, renewables contribute to 90% of the electricity supply in Scotland. Over the first quarter of 2020, clean energy sources contributed to almost half of national electricity generation in the United Kingdom. Similarly, over the first three quarters of the same year, renewables contributed to almost half of Germany’s electricity consumption. Currently, across all the key sectors, i.e. power, heating, industry and transport, demand for renewables is on the rise. The power sector, in particular, is in the lead with its demand for renewables increasing by 8% to reach 8300 TWh; the highest annual growth which was recorded in 2020, according to the IEA (International Energy Agency) [2]. Two-thirds of this amazing renewables growth is attributed to wind and solar PV (photovoltaic); expectedly both sectors are expected to expand in 2021. For instance, over 900 TWh is expected to be generated from these two sources in China this year. These numbers are 589 and 550 TWh for the European Union and the United States, respectively. On global scale, solar PV and wind grew by 12% and 23%, respectively, in 2020. As anticipated, the levelized cost of electricity keeps dropping thanks to technology and market development. However, the capacity factor, for either of solar PV or wind, is too low to allow for baseload energy production with no storage. Battery storage remains essential yet prohibitively expensive. Hence, alternative forms of energy storage including thermal energy storage are called for.

Ultimately, with our move toward energy generation from renewable sources, energy storage remains the key issue. This is mainly because most of the renewable energy sources are intermittent in nature. It can be claimed that a fully renewable energy future is impossible without inexpensive and reliable storage of energy. While chemical, electrical, mechanical and potential energy storage options have been investigated before, the focus of this book is on thermal energy storage in phase change materials (PCMs). Within thermal energy storage techniques, we limit the scope of this book to those relying on phase change. In particular, systems with phase transition from a solid to a liquid state are to be investigated. Applications for such system encompass a wide range including food safety and transport, building energy management, electronic cooling, waste heat recovery, health and well-being, refrigeration, solar thermal power generation, automobile industry, as well as thermal management of satellites. There are certain challenges to be addressed before these PCM-based technologies can be commercialized and we will cover some of those later on. On top of practical questions, there are fundamental research questions to be answered. Some of these questions center on the tools we use to analyze the thermo-chemo-mechanical behavior of thermal storage systems. This book will offer a combination of theoretical, numerical and experimental techniques to address these questions in forthcoming chapters. Moreover, while development of a technology is important, its deployment and market success will depend on a number of parameters among which cost can be investigated and minimized as will be shown in this book.

Thermal energy storage (TES) technologies may be classified according to following four aspects: working temperature, duration of storage, circulation of heat transfer medium and energy storage mechanism [3, 4, 5] (Figure 1.1).

In the sensible heat storage (SHS), heat may be stored in solids (rocks, concrete, bricks, sand, or metals). Heat can also be stored in liquids: most often liquids water, molten salts and mineral oils. In the latent heat storage (LHS), energy is stored, in PCMs, when material changes its state from solid to solid [11], solid to liquid or from liquid to gas while the solid-liquid phase change is the most widely adopted technology, hence the exclusive focus of this book. The frequently used PCMs are organic, inorganic and eutectic materials, as well as metallic liquids which have been studied at laboratory scale [12] (Figure 1.3).

Relying on high latent heat for PCMs is essential. Ideally, a low mass of the PCM should hold as much as possible of latent heat of fusion. The phase change process initiates at a given melting point, and the released heat must be conducted through the solid and liquid phases. In view of the above, these three, melting temperature, latent heat and thermal conductivity, can be considered as the major thermophysical properties of PCMs [13]. Thermophysical properties of some PCMs are presented in Table 1.1 [12] and, in the interest of brevity, only the three major ones are presented here.

Thermochemical storage relies on thermochemical reaction storage and sorption storage. In the thermochemical energy storage system, the thermal energy is first supplied to the thermochemical material, say C, to dissociate it into two products, say A and B. This charging process is an endothermic reaction. Then the two compounds A and B are stored separately. During the discharge process, the two compounds are reunited again and the stored energy is released thanks to an exothermic reaction. The studied thermochemical materials include MgSO₄, FeCO₃, and CaCO₃.

Sorption can be thought of as a process of capturing a gas or a vapor (called sorbate) by a substance (solid or liqui...