Kürklü et al. (1996) developed a model which was validated with an experiment to predict the thermal performance of a square-section PCM store based on conservation of energy. He used Sodium sulphate, sodium chloride with other additives as the PCM and air as the heat transfer fluid. He discovered that as the air mass flow rate increased, there is a corresponding decrease in the phase change time of the PCM (complete freezing). Nallusamy (2016) carried out experiment based research and observed that as the flow rate increases, the time required for complete charging reduces. Using flow rates of 2, 4 and 6 kg/min; he discovered charging time was smaller at 6kg/min. This is because when the flow rate increases, it results in an increase in surface heat transfer coefficient between the HTF and the PCM. This means the flow rate has an effect when charging and discharging.
Esen et al. (1998) investigated the performance of a solar assisted cylindrical energy storage tank which is part of a domestic heating system. For his model, he used the enthalpy method, solving by Gaus-sieldel iteration process. He discovered the cylindrical wall material and radius of tank were not appropriate due to the size of the cylinder. He recommended smaller radii and higher thermal conductivity material which would increase PCM melting time. The effect of the mass flow rate at a given
time affects the amount of stored energy. This is because more heat is removed from HTF, consequently more heat stored in the PCM.
(Agyenim and Hewitt (2012), Agyenim and Hewitt (2010)) carried out an experiment similar to this research using RT58 as the PCM. They used a finned longitudinal shell and tube heat exchanger because of the low thermal conductivity of RT 58 and the advantage of preventing sub-cooling of PCM through nucleation. From his experiment, as the temperature of the inlet HTF temperature increases, so does the heat transfer and the amount of heat energy charged. They increased the inlet HTF from 62.9°C to 76.7°C and concluded that full melting of the PCM occurred at 71°C and 76.7°C and that any temperature below that means complete melting did not take place. Agyenim’s experiment had a bypass valve connected to the hot water bath which is PID controlled, so as to have a set value of inlet temperature before charging starts. This was utilised in this research’s experimental work.
Agyenim and Hewitt (2012) discovered that when RT58 PCM is charged it cannot be melted (has not even reached its phase change zone) within four (4) hours at any value of inlet HTF temperature. He recommends twenty-four (24) hours of charging, ie. a whole day to complete heating of the system. This means the maximum time of charging under economy 7 cannot be achieved; hence it’s important to design an advanced heat exchanger system whereby the PCM can be charged within the period of electricity tariff reduction.
In thermal storage using phase change material, using multiple PCMs (m-PCM) has enhanced the performance of the storage unit compared with using only a single PCM. The arrangement of multiple PCMs in decreasing order of their melting temperature during charging results in higher heat transfer rate. Various researchers have experimentally tested the performance. Tian et al. (2012) arrangement of the multiple PCM was in increasing order of melting during charging as shown in Figure 2:5, which is a different approach to other researchers as shown in Figure 2:6. This arrangement would decrease the heat transfer rate when HTF flows from a PCM with a low melting point to one with a higher melting point. Tian’s arrangement will be useful for the discharging process but not the charging process. Tian’s arrangement is not ideal, because the PCM melting temperature was not considered for the cascading storage. It is not possible to have an effective heat transfer with that storage. The right order
should be PCM3, PCM 2 and PCM1 in order to maintain similar temperature differences between the PCMs and HTF.
Figure 2:5: Cascaded thermal storage unit with thermal properties. Tian et al. (2012)
Figure 2:6: Cascaded thermal storage unit. Horst Michels and Pitz-Paal (2006) Wang et al. (2015a) carried out a numerical simulation using three (3) PCMs contained between zig-zag configuration plates as shown in Figure 2:7. The arrangement of the PCM are in the order of PCM1, PCM 2and PCM 3 in the flow direction of the heat transfer fluid with decreasing order of their phase change temperature(PCM-1>PCM- 2>PCM-3). A similar arrangement was investigated by Jegadheeswaran and Pohekar (2009) as shown in Figure 2:8, who used five different PCMs in a shell and tube heat exchanger.
Figure 2:7: : Schematic representation of the zigzag configuration.Wang et al. (2015a).
Figure 2:8:Multiple PCM arrangement in a shell and tube exchanger.Jegadheeswaran and Pohekar (2009).
2.5 Summary
The classification of PCMs and the advantages they possess over each other is discussed in detail. The mode of selection of PCM is discussed and their properties required for various TES application. The types of PCM and geometry used by other researchers are discussed. The various ways PCM can be enhanced by cascading, modifying the geometry (use of fins) and encapsulation is discussed in this chapter. The cascading method is used to give a more uniform temperature difference between HTF and store such that a more constant heat (charge rate) is maintained and as much energy extracted from the HTF as possible as it applies to enhancing the properties of the PCM as the various types of PCM are arranged according to their thermal properties (with the PCM with the highest melting point arranged in descending order).
The various modelling methods used by other researchers are discussed and the advantages they possess over each other are explained. The type of software used to solve the modelling problem is mentioned. Various ways in which the performance of TES can be enhanced are discussed, with mention of cascading and how to improve the thermal conductivity in PCMs with low thermal conductivity (Paraffins).
3 Phase change materials
3.1 Classification of PCM.
Phase change materials can be classified as organic or inorganic material. They possess the ability to absorb or release energy during change of process. Figure 3:1 shows the phase change cycle undergone by the PCM as it is heated or cooled.
Figure 3:1: Phase change process. Al-Hallaj and Kizilel (2012).