Motoi Yamaha

The Evaluation of Peak Shaving by a Thermal Storage System Using Phase-Change Materials in Air Distribution Systems

An air distribution system equipped with a storage device using phase-change materials (PCMs) was proposed for peak shaving. Mixtures of paraffin waxes were chosen as the PCMs, and experiments were conducted to identify thermal characteristics, such as melting temperature and latent heat. Although the mixtures had less heat storage capacity than the pure wax, the heat of fusion was around 90 kJ/kg (40 Btu/lb), which could be competitive with sensible storage materials. A simulation using the enthalpy method was conducted to evaluate system performance. The PCM was cooled during nighttime, and the stored heat was discharged from 13:00 to 16:00, while the heat source was stopped. The simulation program evaluated temperature fluctuation during the discharge period for each mixture and quantity of PCM. Installing 5 kg/m2 (1 lb/ft2) of PCM, the temperature during the discharge period could be maintained within an acceptable condition. It is concluded that this system could contribute to peak shaving for buildings.

PHASE CHANGE MATERIALS: APPLICATION FUNDAMENTALS

This chapter covers fundamentals for the application of PCM for different systems and products. The chapter starts with an introduction into heat transfer mechanisms by analytical and numerical models. Then different designs for storages are discussed including their advantages and disadvantages with respect to liquids and gases as heat transfer medium. The chapter ends with a presentation of the different PCM measurement technologies. These technologies are DSC for small samples, and T -history method for bigger samples. In situ measurement will also be commented.

Thermal Energy Storage Tanks Using Phase Change Material (PCM) in HVAC Systems

Thermal energy storage (TES) systems, which store energy as heat, can compensate for energy imbalances between heat generation and consumption (Tamblyn, 1977). Thermal energy storage systems designed for use with solar energy can accumulate unstable solar insolation. These systems can also shave the peak heat demand to off-peak hours. As such, the required capacity of refrigeration machines can be reduced by extending the time during which these machines are operated. These system also offer other advantages, such as load leveling for the energy supply side. Electric utility companies in Japan offer discount rates during nighttime to promote peak shaving. Methods of storing heat can be classified as sensible, latent, or chemical storage methods. Sensible heat storage uses heat capacity obtained through a temperature difference. Latent heat storage utilizes the heat to produce a phase change. Considering the associated volume expansion, the phase change between a liquid and a solid is generally used. Reversible chemical thermal reactions can also be used to store heat. For latent heat storage, various phase change materials (PCM) for different temperature ranges have been investigated. Since these materials should be inexpensive, abundant, and safe, water or ice are the most attractive storage materials for use in the heating, ventilation, and air conditioning (HVAC) field. Water has a relatively high heat of fusion and a melting temperature that is suitable for cooling. The freezing point is suitable for comfort cooling, even though the low evaporation temperature of the refrigeration cycle decreases the efficiency of the machine. Since the tank volume is smaller than the water tank, heat losses from the tank are also smaller. Buildings that do not have sufficient space for water storage still can take advantage of TES through the use of ice storage tanks. Although ice storage tanks have been used successfully in commercial applications, their use is limited to cooling applications and lowers the efficiency of refrigeration machines due to the lower evaporative temperature associated with these tanks. Phase change materials other than ice have been studied for various purposes. Paraffin waxes, salt hydrates, and eutectic mixtures are materials for use in building applications. Compared to ice storage, these PCMs are used in a passive manner such as the stabilization of room temperature by means of the thermal inertia of phase change.