Thermal Energy Storage

The following sections are included:

• Preface
• About the Authors
• Contents

The practice of storing thermal energy dates back to ancient civilizations from forms such as storage of ice blocks buried in sawdust and straw, to the use of heated rocks for cooking and warmth in colder climates. Modern-day thermal energy storage, in contrast, has taken on much more sophisticated forms in both domestic and commercial settings, which have become the mainstays in thermal system designs around the world. While simplistic in concept, the nuances in application and design have spawned a wide range of in-depth research, which has led to a rich and diverse field of study in society today. This opening chapter intends on providing a brief overview of the role of thermal energy storage in today’s world. Additionally, the classifications of thermal energy storage will be covered without going into excessive detail. Lastly and most importantly, the various thrusts of research and development today will be discussed.

Sensible thermal energy storage (STES) systems constitute the most established and widely applied form of thermal energy storage in industry. In this chapter, the fundamentals of STES systems as well as the various aspects of their performance analysis will be first covered. Following which, case studies will be delved into and recent innovations and studies involving both commercial and academic fields will be explored.

Latent thermal energy storage (LTES) systems utilize the phase-change properties of the storage media to allow high thermal energy densities to be contained within narrower temperature bands. Latent heat is released or absorbed during a material’s phase-change process and this heat is many times higher than its specific heat capacity while occurring at a near-constant temperature. This allows for higher thermal energy densities for applications in contrast to sensible TES systems. In the ensuing sections of this chapter, the fundamentals of PCMs and the thermal transfer of LTES systems will be first covered. Second, the various classifications and manifestations of LTES systems will be introduced. Finally, several real-life examples of LTES applications will be described to illustrate the increasing impact that this form of TES has on the dynamic energy distribution landscape.

Renewable energy sources coupled with heating and cooling applications serve as great connectors between thermal supply and demand. The previous chapters have demonstrated that sensible and latent thermal energy storage systems could be applied to in situ heat transfer and energy storage applications. Latent energy storage systems offer around 5–15 times higher energy storage density than sensible energy storage systems, thereby making them more compact. Principally different from sensible and latent energy storage technologies, thermochemical energy storage systems operate on reversible chemical reactions and store energy in the employed material’s chemical potential. As a result, this technology offers up to 10 times more improvement in energy storage density than the latent energy storage systems. Since energy storage occurs based on the material’s chemical potential, it can be stored even in ambient conditions without significant losses as long as the reactants remain separated. This characteristic offers considerable advantages in addressing the challenges associated with energy storage. This chapter discusses the fundamental operating principle of different thermochemical reactions and provides a comprehensive overview of two types of thermochemical processes: sorption and reaction-driven energy storage systems. Different designs and configurations are discussed, ideal characteristics of these materials are identified, and key challenges associated with improving this technology’s readiness levels are thoroughly described.

The key purpose of a TES unit is simply to perform as a time-shifting buffer between peak supply and demand periods over varying timescales. However, the integration of TES units into a thermal operating system is non-trivial and involves several issues, including (1) How should they be sized? (2) How should they be optimized to suit the needs of the end user? and (3) How should they be incorporated into a system in order to promote the overall system performance? This chapter describes system-level TES integration and the various approaches taken by the industrial and academic fields to incorporate TES as an integral part of system-level operation. Fundamental principles such as the system operating design and control will be first covered, followed by recent and more niche applications in the various industrial sectors.

As large-scale district cooling systems become more complex, the level of tediousness behind designing and applying TES systems increases. This is expected as the TES concept is largely centred on a static thermal capacitance used to balance mismatching supply and demand in thermal processes. Several key questions remain. First, how should the field continue to evolve to fit itself into the next-generation of thermal distribution landscapes? What directions would be necessary for TES research to stay relevant and applicable? This last chapter aims to provide some perspectives on these questions.

The following section is included:

• Index