**2.2. Phase change materials**

PCMs are substances that can absorb, store and release a large amount of thermal energy at relatively constant temperatures and are well suited for heat transfer and energy conservation applications [3, 8]. Over the last several decades, different types of PCMs with different melting temperatures and heat of fusion, as shown in **Figure 6** [9, 10], have been developed. PCMs can be generally grouped into three categories based on their chemical composition, including organic compounds, inorganic compounds, and inorganic eutectics or eutectic mixtures [11].

PCMs as thermal energy storage (TES) should possess desirable thermophysical, kinetic and chemical properties. To characterise the thermophysical properties of PCMs, thermal conductivity analyser and differential scanning calorimetry (DSC) are often used to measure the thermal conductivity and the phase change temperature/heat of fusion of PCMs, respectively. **Figure 7** presents an example of the DSC test results of a commercial PCM product of RT24 [12] under different scanning rates of 0.5, 0.3 and 0.1 K/min, respectively.

PCMs can be incorporated into building envelopes to increase thermal mass and reduce indoor temperature fluctuations as well as to reduce/shift the building heating and cooling demand. PCMs can also be integrated into building HVAC systems as centralised thermal energy storage units to enhance the operating efficiency of HVAC systems through effective load management.

**Figure 4.** Illustration of the laboratory-scale PVT test rig.

Solar-Assisted HVAC Systems with Integrated Phase Change Materials http://dx.doi.org/10.5772/intechopen.72187 25

**Figure 5.** Thermal energy and electricity generation as well as electrical efficiency of the PVT collectors under the two summer test days.

**Figure 6.** Melting temperature and heat of fusion of different PCMs [9, 10].

**2.2. Phase change materials**

24 Sustainable Air Conditioning Systems

load management.

**Figure 4.** Illustration of the laboratory-scale PVT test rig.

PCMs are substances that can absorb, store and release a large amount of thermal energy at relatively constant temperatures and are well suited for heat transfer and energy conservation applications [3, 8]. Over the last several decades, different types of PCMs with different melting temperatures and heat of fusion, as shown in **Figure 6** [9, 10], have been developed. PCMs can be generally grouped into three categories based on their chemical composition, including organic compounds, inorganic compounds, and inorganic eutectics or eutectic mixtures [11]. PCMs as thermal energy storage (TES) should possess desirable thermophysical, kinetic and chemical properties. To characterise the thermophysical properties of PCMs, thermal conductivity analyser and differential scanning calorimetry (DSC) are often used to measure the thermal conductivity and the phase change temperature/heat of fusion of PCMs, respectively. **Figure 7** presents an example of the DSC test results of a commercial PCM product of RT24

**Figure 3.** Weather condition and PVT inlet and outlet air temperatures under the two summer test days.

PCMs can be incorporated into building envelopes to increase thermal mass and reduce indoor temperature fluctuations as well as to reduce/shift the building heating and cooling demand. PCMs can also be integrated into building HVAC systems as centralised thermal energy storage units to enhance the operating efficiency of HVAC systems through effective

[12] under different scanning rates of 0.5, 0.3 and 0.1 K/min, respectively.

**Figure 7.** DSC test results of PCM RT24 with the scanning rates of 0.5, 0.3 and 0.1 K/min, respectively.

**Figure 8** presents the charging and discharging performance of an air-based PCM thermal energy storage unit, which was tested based on a laboratory-scale rig (**Figure 9**) when the air flow rate was 100 L/s. The PCM tested was a commercial product of PCM S21 [13]. It can be seen that at the beginning of the charging process, both the air temperatures at the inlet and outlet of the PCM TES unit increased rapidly. Then, the outlet air temperature (measured by the temperature sensor #5) increased gradually until approaching to the inlet air temperature of 42°C at the end of the charging process, and the PCM in the TES was melted into the liquid phase during this process. During the discharging process, the outlet air temperature (measured by the temperature sensor #2) from the PCM TES unit first decreased and then slightly increased due to supercooling of the PCM and then continuously decreased to around 14°C at the end of the discharging process. It is worthwhile to mention that, during the experimental tests, the air flow directions in the charging mode and discharging mode were opposite in order to ensure a good heat transfer performance during the discharge of the PCM TES unit.

**3. Overview of solar-assisted HVAC systems with integrated PCMs**

thermal energy regulation and space conditioning.

tion of the auxiliary heater by about 31%.

accounted for 7.11% of the total coolness stored.

**Figure 10.** Scope of the review.

Many different solar-assisted HVAC systems such as solar-driven ejector air conditioners [14], direct current air conditioning systems integrated with photovoltaic (PV) systems [15], solardriven absorption air conditioning systems [16] and solar-assisted desiccant cooling systems [17, 18] have been developed over the last two decades. As shown in **Figure 10**, in this section, the review mainly focuses on the solar-assisted HVAC systems with integrated PCMs for

Solar-Assisted HVAC Systems with Integrated Phase Change Materials

http://dx.doi.org/10.5772/intechopen.72187

27

A solar-driven adsorption cooling system with an integrated PCM TES unit was investigated by Poshtiri and Jafari [19] in order to provide 24-h air conditioning. The system consisted of an adsorption cooling system, solar collectors, a water storage tank, a PCM TES unit and an auxiliary heater. The thermal energy generated by the solar collectors can be either used to power the adsorption cooling system or stored in the PCM TES unit. The solar energy stored in the PCM TES unit during the daytime was used to power the adsorption cooling system during the night-time if needed. The simulation results showed that the hourly electricity consumption of this system was approximately 30% less than that of a conventional air conditioner during the daytime, and the PCM TES unit reduced the night-time energy consump-

A solar adsorption cooling system integrated with a PCM cold storage system was studied by Zhai et al. [20]. When sunshine was sufficient, the PCM TES unit will be charged using the regenerated coolness from the adsorption chiller. The cooling energy in the PCM can be discharged for space cooling through a radiant cooling terminal unit during the night-time. The results from the experimental test showed that the average charging and discharging rates of the PCM TES unit were 56.7 W and 79.1 W, respectively. The cooling capacity loss only

A solar heating and cooling system with an absorption chiller and a compact PCM TES unit with capillary tubes was proposed by Helm et al. [21]. Through integrating the PCM TES unit

Solar energy using PCMs

Air conditioning

Energy storage

**Figure 8.** Air temperatures at the inlet and outlet of the PCM TES unit.

**Figure 9.** Laboratory-scale test rig of the PCM TES system.
