8.3 Applications of phase change materials for sustainable energy  (Page 33/26)

Learning objectives

After reading this module, students should be able to

• learn the general concept of Phase Change Materials (PCM)
• understand the applications of PCMs in sustainable energy
• recognize the uses of PCM for heating and cooling systems
• recognize the uses of PCM in buildings
• recognize the uses of PCM in transportation

Introduction

The growing demand for sustainable energy from consumers and industry is constantly changing. The highest demand of energy consumption during a single day brings a continuous and unsolved problem: how to maintain a consistent desired temperature in a sustainable way. Periods of extreme cold or warm weather are the triggering factors for increasing the demand on heating or cooling. Working hours, industry processes, building construction, operating policies, and type and volume of energy production facilities are some of the main reasons for peak demand crises. Better power generation management and significant economic benefit can be achieved if some of the peak load could be shifted to the off peak load period. This can be achieved by thermal storage for space heating and cooling purposes.

Thermal energy can be stored as a change in the internal energy of certain materials as sensible heat    , latent heat    or both. The most commonly used method of thermal energy storage is the sensible heat method, although phase change materials (PCM), which effectively store and release latent heat energy, have been studied for more than 30 years. Latent heat storage can be more efficient than sensible heat storage because it requires a smaller temperature difference between the storage and releasing functions. Phase change materials are an important and underused option for developing new energy storage devices, which are as important as developing new sources of renewable energy. The use of phase change material in developing and constructing sustainable energy systems is crucial to the efficiency of these systems because of PCM’s ability to harness heat and cooling energies in an effective and sustainable way.

Phase change materials for energy storage devices

Thermal storage based on sensible heat works on the temperature rise on absorbing energy or heat, as shown in the solid and liquid phases in Figure Temperature Profile of a PCM . When the stored heat is released, the temperature falls, providing two points of different temperature that define the storage and release functions. Phase change materials are conceptually different, however. They operate by storing energy at a constant temperature while phase change occurs, for example from solid to a liquid, as illustrated in the center of Figure Temperature Profile of a PCM . As heat is added to the material, the temperature does not rise; instead heat drives the change to a higher energy phase. The liquid, for example, has kinetic energy of the motion of atoms that is not present in the solid, so its energy is higher. The higher energy of the liquid compared to the solid is the latent heat . When the solid is fully transformed to liquid, added energy reverts to going into sensible heat and raising the temperature of the liquid.