Solidification and Melting of Phase Change Material in Cold Thermal Storage Systems

*Hani Hussain Sait*

## **Abstract**

Cold thermal storage can be used to manage peak load when the energy demand is exceeding the capacities of the electric companies. Latent heat thermal storage is more effective because it requires less spacing and has higher thermal capacity than other types. Solidification and melting are taking place in CTS and need more investigation for better performance. Phase change materials properties vary and need more investigation to select the most suitable for a certain application. The analytical equations are needed for design of CTS and get the maximum efficiency out of it. Cost effectiveness is also described.

**Keywords:** solidification, melting, cold thermal storage

## **1. Introduction**

The electric energy consumption in a country like Saudi Arabia reaches its peak during summer time. Most of this consumption goes to air conditioning, i.e. 75% to 90% of total electric energy production by the Saudi Electrical Companies during summer season are consumed for cooling. This put an excessive load on the electricity suppliers during summer time, at which a peak load exists as shown in **Figure 1**. The peak load or Peak demand are terms used in energy demand management describing a period in which electrical power is expected to be provided for a sustained period at a significantly higher than average supply level.

Peak load occurs usually during the day in hot countries. Most of the energy was consumed by the air conditioning for residential and commercial buildings. Load management initiatives are usually investigated by the electric companies to smooth the system load curve (Load leveling of electricity) (**Figure 1**). Many methods are suggested to handle this problem, such as utilizing renewable energy, unfixed Tarriff, use of the electrical link, and finally utilizing of energy storage systems.

Cold thermal storages were built successfully in several projects in Saudi Arabia such as, Al Mamlaka Tower in Riyadh, King Khalid Training Center in Riyadh and King Abdulaziz University Campus in Jeddah.

#### **1.1 Concept of thermal energy storage**

Unique solution to manage the peak load, that can save energy and will not cost so much is the storing of energy. The different forms of energy that can be stored include mechanical, electrical and thermal energy. Note that the Energy storage is

**Figure 1.** *Load distribution during the day (load leveling of electricity), [1].*

not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems and plays an important role in conserving the energy. Thermal energy can be stored during the un- peak period, usually at night, and re-use it during the peak load.

TES technology can reduce the generating and operating costs of cooling plant equipment. By utilizing TES, new generating plants can be eliminated. Moreover, some electric companies initiated different Tariff rate to reduce the use of electricity during the peak demands and force big consumers to store energy in the offpeak time by utilizing TES. In applications where peak loads occur only for a limited period during a year, such as worshipping places, which are used only for couple of hours during the day or the weak, TES systems can also be used, so that it can be to store the full need of cooling energy with reduced size equipment.

Thermal energy storage can be stored as a change in internal energy of a material as sensible heat, latent heat and thermochemical or combination of these. Cold storage technology has improved significantly since 1980 when electric utility companies recognized the need to reduce the peak demand on their generation and distribution systems. Chilled water, ice, or eutectic phase change materials are the cold storage media.

#### **1.2 Classification of cold thermal storage**

Cold thermal storage systems which are classified into thermal or chemical. The thermal CTS is classified into Sensible or latent heat storage system as shown in **Figure 2**.

#### *1.2.1 Chilled water storage systems*

Among all types of liquid water is selected to be the thermal storage medium, since it has the highest specific heat of all common materials (4.18 kJ/kg.°C). Chilled water with temperature 5 °C -7 °C can be generated by conventional chiller unites. The chilled water is then can be stored in an isolated concrete or stainless-steel tank for later use to meet the cooing needs, **Figure 3**. In general, in order to store 1 kWh of energy, a volume of approximately 0.1 m3 is required. The chilled water can be pumped to the air handling unite (AHU) from the bottom of the tank and at the same time is substitute from the top by the return warm water from the AHU. Stable layers of water can be achieved due to variation of densities according on the temperature. This type of CTS is cost effective when the space is available. The chilled water tank by itself has other uses such as a back up water reservoir or for emergency fire extinguisher.

*Solidification and Melting of Phase Change Material in Cold Thermal Storage Systems DOI: http://dx.doi.org/10.5772/intechopen.96674*

**Figure 2.** *Thermal energy storage types.*

The quantity of stored heat in the storage tank can be calculated by

$$Q = \dot{m} \mathbf{C}\_p (T\_\epsilon - T\_i) \tag{1}$$

Where m is the mass of the chilled water,

#### *1.2.2 Latent heat storage*

When the materials go for a phase change from solid to liquid, liquid to gas or vice versa, it can store or release huge amount of heat which is latent heat. Latent heat storage (LHS) is getting more attractive because of the huge amount of energy that can be stored with small space which is usually one fourth less than the chilled water storage system. **Figure 4** shows the latent heat storage mechanism for solid liquid phase change.

**Figure 5** shows a thermal storage system that utilizes ice. The size of the storage tank depends on the total volume of the melted liquid.

On the other hand, the ice storage system has COP of 2.5–4.1 which is less than that of chilled water storage system of 5–5.9 COP. So that the ice storage system economic benefit is beneficial for less Tarif at the off-peak time.

The storage capacity of the LHS system with a PCM medium is given by

$$\mathbf{Q} = \int\_{T\_i}^{T\_\epsilon} \dot{m} \mathbf{C}\_p dT + \dot{m} a\_{fr} \Delta h + \int\_{T\_\epsilon}^{T\_{fr}} \dot{m} \mathbf{C}\_p dT \tag{2}$$

Or:

$$\mathcal{Q} = \dot{m} \left[ \mathbf{C}\_p (T\_e - T\_i) + \mathbf{a}\_{fr} \Delta h + \mathbf{C}\_p \left( T\_{fr} - T\_e \right) \right] \tag{3}$$

The specific volumetric storage capacity of ice stores is 40–53 kWh m<sup>3</sup> [2].

Ice storage systems allow for innovative HVAC system design such as cold air distribution systems which have lower initial costs compared to conventional distribution systems. Ice storage systems include: Ice harvesters, Internal melt ice-oncoil storage systems, External melt ice-on-coil storage systems and Containerized ice storage systems. More details on each one of them can be found in Moncif Karaci "Energy Audit of Building System: An Engineering Approach".

#### *1.2.3 Thermochemical energy storage*

By a reversible chemical reaction, the energy can be absorbed or released for a thermochemical system. The stored thermochemical heat relies on the amount of storage material, the endothermic heat of reaction, and the extent of conversion.

#### **Figure 4.**

*Latent heat storage mechanism for solid –liquid.*

*Thermal storage system that utilizes ice, the size of the storage tank depends on the total volume of the melted liquid.*

*Solidification and Melting of Phase Change Material in Cold Thermal Storage Systems DOI: http://dx.doi.org/10.5772/intechopen.96674*

#### *1.2.4 Eutectic systems*

Salt and oil are two types of eutectic material. A eutectic salt can change its phase from liquid to solid at a specific temperature. These phases may have different crystal structures, or the same crystal structure with different lattice parameters. The phase change material such as Eutectic salts can have a freezing point of 8.5 °C which means less consumption of energy than ice. The PCM used in Eutectic system has less capacity than ice storage system but higher capacity than chilled water system. The eutectic system is more expensive and complex than chilled water systems and has warmest discharge temperatures (near 10 °C). More details of PCM materials can be found with [3].

#### **1.3 Phase change materials**

Phase change materials (PCM) are "Latent" heat storage materials. Unlike conventional (sensible) storage materials, PCM absorbs and release heat at a nearly constant temperature. They store 5–14 times more heat per unit volume than sensible storage materials such as water, masonry, or rock. PCM can be classified as organic, inorganic or eutectic which can be available in any required temperature range. The inorganic materials have almost double volumetric latent heat storage capacity (250–400 kg/dm3) than the organic materials (128–200 kg/dm3).

Organic materials are further described as paraffin and non-paraffins. Organic materials include congruent melting means melt and freeze repeatedly without phase segregation and consequent degradation of their latent heat of fusion, self-nucleation means they crystallize with little or no supercooling and usually non-corrosiveness. Inorganic materials are further classified as salt hydrate and metallics. These phase change materials do not supercool appreciably and their heats of fusion do not degrade with cycling.

The melting point of Paraffin varies between 5.5 °C and up to 75.9 °C, [4]. The latent heat of fusion varies from 170 kJ/kg and up to 269 kJ/kg. For the nonparaffin, the melting points also varies from 7.9 °C to 127.2 °C. They have heat of fusion ranging from 93 kJ/kg to 259 kJ/kg. The non-paraffin materials will have its own properties unlike the paraffin's, which have very similar properties. For the inorganic materials has a melting point varies from 16.7 to 102 and heat of fusion ranges from 146 to 242 kJ/kg. The high melting point allows les energy needed for those PCM to change state. The relatively high heat of fusion is another factor can affect the selection of such materials. **Table 1** shows the properties need to be considered in PCM. More details about the recommended PCM can be found in [4–6].


#### **Table 1.** *Required features for the PCM materials.*
