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

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 thermal energy regulation and space conditioning.

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 consumption of the auxiliary heater by about 31%.

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 accounted for 7.11% of the total coolness stored.

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

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

**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

> Time (h) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

PCM Charging PCM Discharging

Charging,*Ta,in* Charging,*Ta,out* Discharging,*Ta,out* Discharging,*Ta,in*

the PCM TES unit.

26 Sustainable Air Conditioning Systems

Temperature (°C)

12

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

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

into the heat rejection system of the absorption chiller, a fraction of the rejected heat of the absorption chiller can be buffered under the cooling operation, which can allow a lower coolant temperature during the peak demand period, leading to an increased system efficiency. The heat stored in the PCM can be discharged during night-time or off-peak periods. The pilot running of this system showed that, during the hot ambient conditions, up to 50% of the daily rejected heat load can be covered by the PCM TES unit. The overall electrical coefficient of performance (COP) including the regeneration of the PCM TES unit during the night-time was varied between 4.5 and 8.0.

A space heating system with integrated solar air heaters and a fluidised bed TES unit using PCMs was proposed by Belmonte et al. [28]. The use of PCM fluidised bed enabled faster charging and discharging of the TES unit. The results showed that this system can supply approximately 50% of the heating demand of a single-family house under Barcelona and

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The performance of a roof-integrated solar heating system using a PCM TES unit was investigated by Saman et al. [29]. The existing roof was used as a solar air heater. A PCM TES unit was used to store the thermal energy generated during the daytime, and the heat stored in the PCM was discharged during the night-time or when there was no sunshine. The simulation results showed that the effect of sensible heat cannot be neglected, and a higher inlet air

A PCM-enhanced house with the PVT ventilation for space heating and cooling was developed by Lin et al. [30]. The PCM was embedded into building envelopes to increase local thermal mass, and PVT collectors were used to generate both electricity and the low-grade thermal energy for space conditioning. The simulation results showed that using the PCM in the building envelope reduced the indoor temperature fluctuations, and the combination of the PVT ventilation and PCM can substantially increase the indoor thermal performance of the house. A ceiling ventilation system integrated with solar PVT collectors and PCMs was proposed by Lin et al. [31] (see **Figure 11**). The PVT collectors were used to generate electricity and provide the low-grade heating and cooling energy for buildings by using winter daytime solar radiation and summer night-time sky radiative cooling, respectively. The PCM was integrated into the building ceiling as part of the ceiling insulation and at the same time, as a centralised thermal energy storage to temporally store the thermal energy collected from the PVT collectors. The simulation results carried out based on a Solar Decathlon house showed that the thermal performance of the house under the heating conditions was improved significantly by using this system, in comparison with the original house without using PVT collectors and PCMs

Madrid weather conditions when the TES unit used 2000 kg of the granular PCM.

temperature and a higher air flow rate can reduce the melting time.

and the house using the PCM only but without using the PVT collectors.

**Figure 11.** PCM-enhanced ceiling ventilation system with PVT collectors [31].

A solar-driven ejector cooling system with an integrated PCM cold TES unit was proposed by Allouche et al. [22]. A simulation study was performed based on an office with a total floor area of 25 m2 under Tunisian summer weather conditions. The results showed that the use of the PCM TES unit significantly improved the cooling cycle COP of the ejector cooling system and the solar thermal ratio by up to 100%.

Eicker and Dalibard [23] developed a PVT system for night-time radiative cooling of buildings. The system consisted of water-based PVT collectors, a PCM ceiling, an activated floor and a reversible heat pump. During the night-time, the main priority of the PVT collectors was to regenerate the PCM ceiling and then to cool down a storage tank, which was used as a heat sink of the reversible heat pump. The results showed that the PVT-driven PCM ceiling can cover 27% of the total building cooling demand.

An active and responsive solar façade module with integrated PV panels and PCMs was developed by Favoino et al. [24]. In summer, the PCM was used as a passive thermal storage, and a ventilated cavity was used as an outdoor air curtain. In winter, the PCM was used as an active latent heat TES by heating the PCM through electric heat foils when the heating demand was low, and the stored thermal energy was used later when heating demand increased. The ventilated cavity can also be used to preheat the ventilation air in winter. The results showed that this system was able to prevent excessive heat gains in summer and to preheat fresh air for ventilation purposes in winter.

A heat pump with integrated solar collectors and a PCM TES unit was developed by Aydin et al. [25]. The thermal energy generated by the solar collectors was stored in the PCM unit through a closed cycle. The PCM TES unit was then used as the heat sink of the heat pump or as a heat source of hot water for space heating. The results showed that the first law efficiencies of the solar collectors and the PCM TES unit were 70.4% and 74%, respectively, while the second law efficiencies were 2.5% and 37%, respectively. It was also found that the quality of thermal energy was significantly decreased due to the entropy generation.

A solar hot water-driven low-temperature radiant floor heating system consisting of PCMs and polyethylene coils/capillary mat was evaluated by Zhou and He [26]. It was found that the use of the PCM reduced the temperature variations in the floor structure, in comparison with the use of sand for thermal mass. Compared with polyethylene coils, the capillary mat provided a more uniform vertical temperature distribution. A similar floor heating system with capillary plaits and a macro-packaged PCM layer was investigated by Huang et al. [27]. It was found that the PCM floor was able to release 37677.6 kJ heat within 16 h during the pump-off period for a room with a floor area of 11.02 m2 , which accounted for 47.7% of the energy supplied by solar water.

A space heating system with integrated solar air heaters and a fluidised bed TES unit using PCMs was proposed by Belmonte et al. [28]. The use of PCM fluidised bed enabled faster charging and discharging of the TES unit. The results showed that this system can supply approximately 50% of the heating demand of a single-family house under Barcelona and Madrid weather conditions when the TES unit used 2000 kg of the granular PCM.

into the heat rejection system of the absorption chiller, a fraction of the rejected heat of the absorption chiller can be buffered under the cooling operation, which can allow a lower coolant temperature during the peak demand period, leading to an increased system efficiency. The heat stored in the PCM can be discharged during night-time or off-peak periods. The pilot running of this system showed that, during the hot ambient conditions, up to 50% of the daily rejected heat load can be covered by the PCM TES unit. The overall electrical coefficient of performance (COP) including the regeneration of the PCM TES unit during the night-time

A solar-driven ejector cooling system with an integrated PCM cold TES unit was proposed by Allouche et al. [22]. A simulation study was performed based on an office with a total floor

the PCM TES unit significantly improved the cooling cycle COP of the ejector cooling system

Eicker and Dalibard [23] developed a PVT system for night-time radiative cooling of buildings. The system consisted of water-based PVT collectors, a PCM ceiling, an activated floor and a reversible heat pump. During the night-time, the main priority of the PVT collectors was to regenerate the PCM ceiling and then to cool down a storage tank, which was used as a heat sink of the reversible heat pump. The results showed that the PVT-driven PCM ceiling

An active and responsive solar façade module with integrated PV panels and PCMs was developed by Favoino et al. [24]. In summer, the PCM was used as a passive thermal storage, and a ventilated cavity was used as an outdoor air curtain. In winter, the PCM was used as an active latent heat TES by heating the PCM through electric heat foils when the heating demand was low, and the stored thermal energy was used later when heating demand increased. The ventilated cavity can also be used to preheat the ventilation air in winter. The results showed that this system was able to prevent excessive heat gains in summer and to

A heat pump with integrated solar collectors and a PCM TES unit was developed by Aydin et al. [25]. The thermal energy generated by the solar collectors was stored in the PCM unit through a closed cycle. The PCM TES unit was then used as the heat sink of the heat pump or as a heat source of hot water for space heating. The results showed that the first law efficiencies of the solar collectors and the PCM TES unit were 70.4% and 74%, respectively, while the second law efficiencies were 2.5% and 37%, respectively. It was also found that the quality of

A solar hot water-driven low-temperature radiant floor heating system consisting of PCMs and polyethylene coils/capillary mat was evaluated by Zhou and He [26]. It was found that the use of the PCM reduced the temperature variations in the floor structure, in comparison with the use of sand for thermal mass. Compared with polyethylene coils, the capillary mat provided a more uniform vertical temperature distribution. A similar floor heating system with capillary plaits and a macro-packaged PCM layer was investigated by Huang et al. [27]. It was found that the PCM floor was able to release 37677.6 kJ heat within 16 h during the pump-off period for a room

, which accounted for 47.7% of the energy supplied by solar water.

thermal energy was significantly decreased due to the entropy generation.

under Tunisian summer weather conditions. The results showed that the use of

was varied between 4.5 and 8.0.

28 Sustainable Air Conditioning Systems

and the solar thermal ratio by up to 100%.

can cover 27% of the total building cooling demand.

preheat fresh air for ventilation purposes in winter.

with a floor area of 11.02 m2

area of 25 m2

The performance of a roof-integrated solar heating system using a PCM TES unit was investigated by Saman et al. [29]. The existing roof was used as a solar air heater. A PCM TES unit was used to store the thermal energy generated during the daytime, and the heat stored in the PCM was discharged during the night-time or when there was no sunshine. The simulation results showed that the effect of sensible heat cannot be neglected, and a higher inlet air temperature and a higher air flow rate can reduce the melting time.

A PCM-enhanced house with the PVT ventilation for space heating and cooling was developed by Lin et al. [30]. The PCM was embedded into building envelopes to increase local thermal mass, and PVT collectors were used to generate both electricity and the low-grade thermal energy for space conditioning. The simulation results showed that using the PCM in the building envelope reduced the indoor temperature fluctuations, and the combination of the PVT ventilation and PCM can substantially increase the indoor thermal performance of the house.

A ceiling ventilation system integrated with solar PVT collectors and PCMs was proposed by Lin et al. [31] (see **Figure 11**). The PVT collectors were used to generate electricity and provide the low-grade heating and cooling energy for buildings by using winter daytime solar radiation and summer night-time sky radiative cooling, respectively. The PCM was integrated into the building ceiling as part of the ceiling insulation and at the same time, as a centralised thermal energy storage to temporally store the thermal energy collected from the PVT collectors. The simulation results carried out based on a Solar Decathlon house showed that the thermal performance of the house under the heating conditions was improved significantly by using this system, in comparison with the original house without using PVT collectors and PCMs and the house using the PCM only but without using the PVT collectors.

**Figure 11.** PCM-enhanced ceiling ventilation system with PVT collectors [31].

**Figure 12.** Schematic of the solar-assisted HVAC system (where S/A—supply air, O/A—outside air, R/A—return air, E/A—exhaust air, F—fan and D—damper) [32].

An air conditioning system with integrated PVT collectors and a PCM TES unit (see **Figure 12**) was developed by Fiorentini et al. [32] for a Solar Decathlon house. A hybrid model predictive control strategy was also developed to optimise the operation of this system [33]. The experimental results showed that the control strategy developed was capable of effectively managing and optimising the efficiency of this system.

series, was used for both electricity and low-grade thermal energy generation. A glass cover and fins were used to improve the thermal efficiency of the device. The thermal energy collected from the PVT-SAH can be used to drive the desiccant wheel regeneration in cooling conditions or for space heating in heating conditions. The use of such a hybrid system is to achieve a relatively higher air temperature from the PVT-SAH while still maintaining the necessary electricity generation. An air-based PCM TES unit was used to regulate the discrepancy between the thermal energy generated from the PVT-SAH and the thermal energy demand for the desiccant wheel regeneration. A number of PCM arrays were arranged in parallel to create air channels and form the PCM TES unit. A desiccant wheel and an indirect evaporative cooler as well as a heat recovery unit were used to condition the process air.

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**Figure 13.** Schematic of the desiccant cooling system with integrated PVT-SAH and a PCM TES unit.

This system can be used for both daytime and night-time cooling dependent on the building cooling demand. During the daytime, if there is a cooling demand, the heated air from the PVT-SAH will be directly used for the desiccant wheel regeneration. Otherwise, the heated air from the PVT-SAH will be used to charge the PCM TES unit. During the night-time, the heat stored in the PCM TES unit will be used for the desiccant wheel regeneration if there is a cooling demand of the building. It is worthwhile to note that the night-time radiative cooling of the PVT-SAH could be potentially used for space cooling directly. However, such scenario was not considered in this study. During the winter daytime, the heated air from the PVT-SAH can be directly used for space heating or to charge the PCM TES unit. The main potential

operation modes of this proposed system are presented in **Table 1**.

**Figure 14.** Schematic of the hybrid PVT-SAH system.

From the above-mentioned review, it can be seen that solar-assisted HVAC systems with integrated PCMs offered more flexibility to maximise the system operation through the rational utilisation of solar energy. However, the research in this area is far from sufficient, and more prototypes are needed to demonstrate the practical performance of such systems.
