NENK, 106-109

The two phenomena are shown in Figure 2-5.

Figure 2-5 Sensible and latent heat

Another type of energy storage is through thermo-chemical systems. These systems rely on the energy absorbed and released in breaking and reforming molecular bonds in a completely reversible chemical reaction. Thermo-chemical energy storage has not as yet been used in practical applications and both technical and economical questions have yet to be answered for some of the possibilities proposed (Atul Sharma, 2009).

I. Sensible Heat Storage (SHS)

Sensible Heat Systems (SHS) store energy by raising the temperature of a solid or liquid. SHS systems utilize the heat capacity and the change in temperature of the material during the process of heating and cooling. The amount of heat stored depends on the specific heat of the medium, the temperature change and the amount of storage material.

II. Latent Heat Storage (LHS)

Latent Heat Storage (LHS) is based on the heat absorption or release when a storage material undergoes a phase change. Phase change can be in the following form: solid–

solid, solid–liquid, solid–gas, liquid–gas and vice versa. In solid–solid transitions, heat is stored as the material is transformed from one crystalline to another. Solid–solid

Sensible

Sensible Latent Phase change

temperature

Sensible Temperature

Stored heat

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PCMs offer the advantages of less stringent container requirements and greater design flexibility. These transitions generally have small latent heat and small volume changes than solid–liquid transitions. Solid–gas and liquid–gas transition have higher latent heat of phase transition but their large volume changes on phase transition are associated with the containment problems and rule out their potential utility in thermal-storage systems. Large changes in volume make the system complex and impractical. Solid–

liquid transformations have comparatively smaller latent heat than liquid–gas. However, these transformations involve only a small change (of order of 10% or less) in volume.

Solid–liquid transitions have proved to be economically attractive for use in thermal energy storage systems.

Initially, solid–liquid PCMs perform like conventional storage materials; their temperature rises as they absorb heat. As the temperature reaches the PCM’s melting point, it changes phase from solid to liquid. At the range of phase change, the

temperature does not increase even though they continue to absorb heat. This process is reversed when the heat is removed. Due to the regulation in temperature, PCMs store 5 to 14 times more heat per unit volume than sensible storage materials such as water, masonry, or rock (Atul Sharma, 2009). Tay et al. (2012) recently formulated a new measure called the effectiveness-NTU model to calculate the efficiency of PCM and claim it can be as high as 18 times more than ordinary building materials. Figure 2-5 illustrates sensible and latent heat in respect to stored heat and temperature.

Latent heat thermal energy storage is principally attractive due to its ability to provide high-energy storage density and its characteristics to store heat at a small volume change, usually less than 10%. If the container can fit this volume change, then pressure remains constant and consequently the phase change proceeds at a constant temperature corresponding to the phase-transition temperature of PCM (Mehling et al., 2008; Atul Sharma, 2009). They can either capture solar energy directly or thermal energy through natural convection.

2.3.2.2 Transition temperature

Transition temperature ranks 1st in thermo-physical properties studied in PCM literature. As a mark of the importance of this variable, Farid et al. (2004) advises that PCMs should be selected, first based on their transition temperature. It is the study of the temperature at which PCMs change state.

Mehling et al. (2008b) reports that use of PCM in building applications may be for storage and supply or temperature control. A different transition temperature needs to be

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optimized for storage compared to temperature control. The focus for storage and supply is the amount of heat or cool stored and supplied when needed with small temperature change. In applications for temperature control, the focus is on temperature regulation. Two forms of regulation exist in the specification of transition temperature of the PCM to be used for temperature control:

1. The PCM with a melting range at the average room temperature generally buffers temperature fluctuations. This view is corroborated by Xu et al. (2005).

2. The PCM with a higher transition temperature than the average temperature reduces temperature peaks. In this case the PCM disallows the temperature increase or drop above a specified mark.

In a hot climate like Nigeria the latter option is preferable to reduce high temperatures.

Xiao et al. (2009) optimized PCM for energy storage in a passive solar room using a simplified analytic model. The variables examined are the optimal transition temperature, the latent heat capacity to estimate the benefit of the interior PCM for energy storage in Beijing- a humid continental climate. PCM panels were incorporated into the interior surfaces of partition walls, floor and ceiling. The following conclusions are made:

 The equation of the optimal transition temperature, Tm of interior PCM in a lightweight passive solar room is obtained as Equation 2-1:

Equation 2-1

Where T_a is the average room temperature, Q_r is the transmitted solar radiation on the interior surfaces (W), Qr,in is the radiation heat transfer rate from indoor heat sources (W), h_in is the heat transfer coefficient of interior surface (W m⁻²

°C⁻¹), P are the duration (s) and Ain is the area of interior PCM panel (m²)

This formulae indicates that the optimal transition temperature depends on the average indoor air temperature and the amount of radiant energy absorbed by the PCM panels from solar and casual gains.

2.3.2.3 Thickness

The thickness of a chosen PCM product affects heat flux through the PCM. This is important because the performance of PCMs are based on effective melting and

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solidification (Kuznik et al., 2008a). This indicates that the thickness and conductivity are closely linked in the optimization of performance of a given PCM.

The study of the thickness of PCM is based on thermal capacity. Thermal capacity is the ability of a material to store sensible heat depending on the density, specific heat capacity and thermal conductivity of the material. Based on thermal capacity, building construction may be classified into:

I. Lightweight II. Heavyweight

I. Lightweight buildings: Generally, lightweight buildings are made with timber frame and similar light in-fill panels. Conventional wallboard provides little thermal mass (Richardson and Woods, 2008) however when PCM is added to it, the large latent heat attributed to adding PCM to walls is sufficient to reduce interior wall surface temperature fluctuations to just 8% of that of the exterior air.

II. Heavyweight building: On the extreme end heavyweight buildings are made of