8
H. Tyagi et al.
The next part in this monograph, (Part IV) consists of four chapters which deal
with the critical issue of energy storage. Phase change materials (PCM) can be used
for the purpose of storing thermal energy. This allows the end-user to store energy
for those periods where direct solar energy may not be available. Moreover PCMs
can also be used to offset cooling loads for buildings in warmer climatic zones. The
abstracts of each of these four chapters are as follows.
Chapter 11: Efficient storage of solar thermal energy has been a key research area
in recent years. Among the various methods for energy storage, phase change material (PCM) based latent heat systems have shown a lot of promise due to their high
energy storage densities and smaller system sizes. However, the low thermal conductivities of PCM pose a significant challenge in designing such systems, therefore,
augmentation with suitable thermal conductivity enhancers becomes necessary to
improve their energy charging and discharging performances. The use of metal foam
structures embedded in PCM to form composite PCM-metal foam energy storage
system can improve the effective thermal conductivity remarkably due to the high
surface area for heat transfer between the metal foam and the PCM. This chapter
presents a study of PCM-metal foam composite systems for solar energy storage. At
first, a brief overview of the relevant thermal enhancement methods with particular
emphasis on metal foam systems is presented. This is followed by the description of a
typical PCM-metal foam composite system and the important parameters governing
its energy storage performance. Different modelling approaches for such systems and
their advantages and disadvantages are presented. The effect of important factors for
metal foam-PCM composite systems are analyzed by performing pore-scale simulations. It is shown that factors such as metal foam porosity, pore size distribution,
foam material, phase change material and overall system size contribute significantly
towards the melting pattern and energy storage characteristics of these systems.
Chapter 12: In the present work, we propose thermal energy storage by direct
photo-thermal energy conversion (referred to as optical charging) using nanoparticles
laden phase change materials (PCMs). In the conventional thermal storage systems,
the absorbed solar energy is indirectly transferred to the PCM (primarily through
conduction and convection heat transfer mechanisms) and is subsequently stored in
the form of latent heat of the PCM (referred to as thermal charging). Opposed to
the conventional thermal storage strategies; optical charging involves direct interaction of the sunlight with the phase change material (radiation being the predominant
heat transfer mechanism). Broad absorption-based nanoparticles (amorphous carbon)
have been seeded into the pristine phase change material (paraffin wax) to enhance
photo-thermal conversion efficiency. Particularly, we investigate the effect of adding
nanoparticles to conventional PCMs during optical charging process. To understand
the role of nanoparticles; samples of pristine paraffin wax and nano-PCMs [different
concentrations of nanoparticles (0.05, 0.1, 0.2, 0.4, wt%) dispersed in the pristine
paraffin wax] have been optically heated. Furthermore, optical charging has been
compared with the conventional thermal charging process. As per the experimental
observations, the optical charging scheme significantly improves the thermal charging rate (by more than 157%) at optimum nanoparticle concentration (0.2%, in the
present study) as compared to conventional thermal charging.
H. Tyagi et al.
The next part in this monograph, (Part IV) consists of four chapters which deal
with the critical issue of energy storage. Phase change materials (PCM) can be used
for the purpose of storing thermal energy. This allows the end-user to store energy
for those periods where direct solar energy may not be available. Moreover PCMs
can also be used to offset cooling loads for buildings in warmer climatic zones. The
abstracts of each of these four chapters are as follows.
Chapter 11: Efficient storage of solar thermal energy has been a key research area
in recent years. Among the various methods for energy storage, phase change material (PCM) based latent heat systems have shown a lot of promise due to their high
energy storage densities and smaller system sizes. However, the low thermal conductivities of PCM pose a significant challenge in designing such systems, therefore,
augmentation with suitable thermal conductivity enhancers becomes necessary to
improve their energy charging and discharging performances. The use of metal foam
structures embedded in PCM to form composite PCM-metal foam energy storage
system can improve the effective thermal conductivity remarkably due to the high
surface area for heat transfer between the metal foam and the PCM. This chapter
presents a study of PCM-metal foam composite systems for solar energy storage. At
first, a brief overview of the relevant thermal enhancement methods with particular
emphasis on metal foam systems is presented. This is followed by the description of a
typical PCM-metal foam composite system and the important parameters governing
its energy storage performance. Different modelling approaches for such systems and
their advantages and disadvantages are presented. The effect of important factors for
metal foam-PCM composite systems are analyzed by performing pore-scale simulations. It is shown that factors such as metal foam porosity, pore size distribution,
foam material, phase change material and overall system size contribute significantly
towards the melting pattern and energy storage characteristics of these systems.
Chapter 12: In the present work, we propose thermal energy storage by direct
photo-thermal energy conversion (referred to as optical charging) using nanoparticles
laden phase change materials (PCMs). In the conventional thermal storage systems,
the absorbed solar energy is indirectly transferred to the PCM (primarily through
conduction and convection heat transfer mechanisms) and is subsequently stored in
the form of latent heat of the PCM (referred to as thermal charging). Opposed to
the conventional thermal storage strategies; optical charging involves direct interaction of the sunlight with the phase change material (radiation being the predominant
heat transfer mechanism). Broad absorption-based nanoparticles (amorphous carbon)
have been seeded into the pristine phase change material (paraffin wax) to enhance
photo-thermal conversion efficiency. Particularly, we investigate the effect of adding
nanoparticles to conventional PCMs during optical charging process. To understand
the role of nanoparticles; samples of pristine paraffin wax and nano-PCMs [different
concentrations of nanoparticles (0.05, 0.1, 0.2, 0.4, wt%) dispersed in the pristine
paraffin wax] have been optically heated. Furthermore, optical charging has been
compared with the conventional thermal charging process. As per the experimental
observations, the optical charging scheme significantly improves the thermal charging rate (by more than 157%) at optimum nanoparticle concentration (0.2%, in the
present study) as compared to conventional thermal charging.