**4.1. A rotary desiccant cooling system using PVT collectors and PCMs**

Rotary desiccant cooling systems have been considered as one of the alternative approaches to replacing conventional vapour compression systems as such systems do not use chlorofluorocarbons and have the capability of independent temperature and humidity control [34, 35]. Compared to traditional vapour compression systems, rotary desiccant cooling systems are more energy efficient and environmentally friendly [34]. In a rotary desiccant cooling system, the cooling process is achieved by removing the moisture from the process air using a desiccant wheel and reducing the temperature of the process air using evaporative cooling and other cooling technologies.

**Figure 13** presents the schematic of a rotary desiccant cooling system integrated with a hybrid photovoltaic thermal-solar air heater (PVT-SAH) and a PCM TES unit. In this system, the hybrid PVT-SAH (see **Figure 14**), in which the PVT collector and the SAH were connected in Solar-Assisted HVAC Systems with Integrated Phase Change Materials http://dx.doi.org/10.5772/intechopen.72187 31

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

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.

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.

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

**Figure 12.** Schematic of the solar-assisted HVAC system (where S/A—supply air, O/A—outside air, R/A—return air,

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

In this section, two different HVAC systems with integrated PCMs and air-based PVT collec-

Rotary desiccant cooling systems have been considered as one of the alternative approaches to replacing conventional vapour compression systems as such systems do not use chlorofluorocarbons and have the capability of independent temperature and humidity control [34, 35]. Compared to traditional vapour compression systems, rotary desiccant cooling systems are more energy efficient and environmentally friendly [34]. In a rotary desiccant cooling system, the cooling process is achieved by removing the moisture from the process air using a desiccant wheel and reducing the temperature of the process air using evaporative cooling

**Figure 13** presents the schematic of a rotary desiccant cooling system integrated with a hybrid photovoltaic thermal-solar air heater (PVT-SAH) and a PCM TES unit. In this system, the hybrid PVT-SAH (see **Figure 14**), in which the PVT collector and the SAH were connected in

prototypes are needed to demonstrate the practical performance of such systems.

**4. Development of solar-assisted HVAC systems with integrated** 

**4.1. A rotary desiccant cooling system using PVT collectors and PCMs**

managing and optimising the efficiency of this system.

E/A—exhaust air, F—fan and D—damper) [32].

30 Sustainable Air Conditioning Systems

**PCMs**

tors are presented.

and other cooling technologies.


**Table 1.** Operation models of the desiccant cooling system with the PVT-SAH and PCM TES unit.

The performance of this system was evaluated based on a simulation system developed using TRNSYS [36]. The building load was calculated based on a DesignBuilder [37] model reported in a previous study [38]. The details about the models used can be found in Ref. [39]. **Figure 15** illustrates the simulated performance of this system under five consecutive Brisbane (Australia) summer working days when the system was operated under the operation modes III and IV. In this test, it was assumed that, during the working days, the house was occupied from 17:00 to 8:00 next day, and the cooling was provided if needed. Under the operation mode III, the ambient air was heated by the PVT-SAH and then used for charging the PCM TES unit during the daytime, and the charging process was suspended if the outlet air temperature of the PVT-SAH was lower than the average surface temperature of the PCM bricks in the TES unit. Under the operation mode IV, the PCM TES unit was discharged using the preheated air from the heat recovery unit. The outlet air from the PCM TES unit was then used for the desiccant wheel regeneration, and the electric heater was used if the outlet air temperature was lower than the desiccant wheel regeneration temperature (i.e. 65°C) required. At the process air side, the return air from the indoor space was first mixed with the fresh air and then used as the process air in order to improve the system efficiency. The results from **Figure 15** showed that the supply air temperature and humidity ratio can be generally controlled below 20°C and 0.008 kg/kg dry air, respectively. During the majority of the test period, the heat from the PVT-PCM system satisfied the heat required for the desiccant wheel regeneration. The total contribution of the solar energy for the desiccant wheel regeneration during the whole test period was 96.5%.

a PVT fan, a PCM fan and an air source heat pump. The PVT collectors were used to generate electricity and low-grade thermal energy simultaneously. The two PCM layers with an air channel between them were integrated into the building ceiling to increase the local thermal mass and to serve as a centralised TES unit to temporarily store the thermal energy collected

**Figure 15.** Simulation results of the rotary desiccant cooling system using PVT collectors and PCMs. (a) Ambient air and supply air conditions of the desiccant cooling system. (b) Energy required for desiccant wheel regeneration and energy

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33

As shown in **Figure 17**, the system can operate under different modes depending on the weather conditions, the thermal energy stored in the PCM and the indoor heating demand through ON/OFF control of the dampers of D1-D8, the PVT fan, the PCM fan and the air source heat pump system. The details of the operation modes are summarised in **Table 2**. During the daytime, the heated hot air from the PVT collectors can be directed into the PCM TES unit for thermal energy charging by switching on the PVT fan and opening the dampers D2 and D3 or it can be exhausted directly by switching on the PVT fan and opening the damper D1. The thermal energy stored in the PCMs can be used to facilitate the space heating via preheating the return air for the indoor unit of the air source heat pump by switching on the PCM fan and opening the dampers D4, D6 and D8 or could directly be used for space heating by switching on the PCM fan and opening the dampers D4, D6 and D7. In the normal air conditioning mode, only the air source heat pump will be used to maintain the indoor

from the PVT collectors for later use to facilitate the indoor space heating in winter.

supplied by the PVT-PCM system.

#### **4.2. A heat pump system with integrated PVT collectors and PCMs**

The schematic of a heat pump system with integrated PVT collectors and PCMs laminated onto the building ceiling is shown in **Figure 16**. This system was mainly designed for winter space heating, as the main benefit of using the low-grade thermal energy derived from the PVT collectors is for space heating, although the night-time radiative cooling effect of PVT collectors in summer can also be used to improve the energy performance of the proposed system for indoor space cooling. The system consisted of the PVT collectors, two PCM layers,

The performance of this system was evaluated based on a simulation system developed using TRNSYS [36]. The building load was calculated based on a DesignBuilder [37] model reported in a previous study [38]. The details about the models used can be found in Ref. [39]. **Figure 15** illustrates the simulated performance of this system under five consecutive Brisbane (Australia) summer working days when the system was operated under the operation modes III and IV. In this test, it was assumed that, during the working days, the house was occupied from 17:00 to 8:00 next day, and the cooling was provided if needed. Under the operation mode III, the ambient air was heated by the PVT-SAH and then used for charging the PCM TES unit during the daytime, and the charging process was suspended if the outlet air temperature of the PVT-SAH was lower than the average surface temperature of the PCM bricks in the TES unit. Under the operation mode IV, the PCM TES unit was discharged using the preheated air from the heat recovery unit. The outlet air from the PCM TES unit was then used for the desiccant wheel regeneration, and the electric heater was used if the outlet air temperature was lower than the desiccant wheel regeneration temperature (i.e. 65°C) required. At the process air side, the return air from the indoor space was first mixed with the fresh air and then used as the process air in order to improve the system efficiency. The results from **Figure 15** showed that the supply air temperature and humidity ratio can be generally controlled below 20°C and 0.008 kg/kg dry air, respectively. During the majority of the test period, the heat from the PVT-PCM system satisfied the heat required for the desiccant wheel regeneration. The total contribution of the solar energy for the desiccant wheel regeneration during the whole test period was 96.5%.

reach the required setting

Mode III: TES charging The heated air from the PVT-SAH is used for charging the PCM TES unit if there is no demand during the daytime

cooling demand during night-time Mode V: space heating The heated air from the PVT-SAH is directly used for space heating or is

**Table 1.** Operation models of the desiccant cooling system with the PVT-SAH and PCM TES unit.

The heated air from the PVT-SAH will be directly used for desiccant wheel regeneration. The electric heater will be used if the air temperature does not

A fraction of the heated air from the PVT-SAH will be used for the desiccant

The PCM TES unit is discharged for desiccant wheel regeneration if there is a

directed into the TES unit (note: PCM with a different melting temperature should be used). The heat in the PCM can be used later for space heating

wheel regeneration, and the rest will be used for TES charging

**Operation mode Description**

Mode I: regeneration with PVT-

32 Sustainable Air Conditioning Systems

Mode IV: regeneration with TES

Mode II: regeneration with PVT-SAH direct supply and TES

SAH direct supply

charging

discharging

**4.2. A heat pump system with integrated PVT collectors and PCMs**

The schematic of a heat pump system with integrated PVT collectors and PCMs laminated onto the building ceiling is shown in **Figure 16**. This system was mainly designed for winter space heating, as the main benefit of using the low-grade thermal energy derived from the PVT collectors is for space heating, although the night-time radiative cooling effect of PVT collectors in summer can also be used to improve the energy performance of the proposed system for indoor space cooling. The system consisted of the PVT collectors, two PCM layers,

**Figure 15.** Simulation results of the rotary desiccant cooling system using PVT collectors and PCMs. (a) Ambient air and supply air conditions of the desiccant cooling system. (b) Energy required for desiccant wheel regeneration and energy supplied by the PVT-PCM system.

a PVT fan, a PCM fan and an air source heat pump. The PVT collectors were used to generate electricity and low-grade thermal energy simultaneously. The two PCM layers with an air channel between them were integrated into the building ceiling to increase the local thermal mass and to serve as a centralised TES unit to temporarily store the thermal energy collected from the PVT collectors for later use to facilitate the indoor space heating in winter.

As shown in **Figure 17**, the system can operate under different modes depending on the weather conditions, the thermal energy stored in the PCM and the indoor heating demand through ON/OFF control of the dampers of D1-D8, the PVT fan, the PCM fan and the air source heat pump system. The details of the operation modes are summarised in **Table 2**. During the daytime, the heated hot air from the PVT collectors can be directed into the PCM TES unit for thermal energy charging by switching on the PVT fan and opening the dampers D2 and D3 or it can be exhausted directly by switching on the PVT fan and opening the damper D1. The thermal energy stored in the PCMs can be used to facilitate the space heating via preheating the return air for the indoor unit of the air source heat pump by switching on the PCM fan and opening the dampers D4, D6 and D8 or could directly be used for space heating by switching on the PCM fan and opening the dampers D4, D6 and D7. In the normal air conditioning mode, only the air source heat pump will be used to maintain the indoor

**Figure 16.** Schematic of the heat pump system with integrated PVT collectors and PCMs.

thermal comfort through opening the dampers D5 and D8. It should be noted that the PVT direct heating mode may cause overheating during the daytime. However, under some cold weather conditions, the direct heating mode could be considered.

The performance of this system was evaluated through numerical simulation. The heating demand of the house was simulated using TRNSYS, and the indoor thermostat setting was specified according to NatHERS [40]. A two-layer PCM TES model (**Figure 18**) and a PVT model developed in a previous study [41] were used to facilitate the performance simulation of the proposed system. The governing equations for the energy balance of the PCM layers and the fluid air in the PCM TES unit are described in Eqs. (1) and (2), respectively. In the model of the PVT collector, six nodes were vertically discretised, including the glass cover, PV plate, absorber plate, fins, fluid air and the bottom plate. The governing equations for the glass cover and the fluid air are described in Eqs. (3) and (4), respectively, while the governing equations of the other nodes can be described in the same way.

$$
\rho\_{\rm PCM} \frac{\partial h\_{\rm PCM}}{\partial t} = \dot{k}\_{\rm PCM} \frac{\partial^2 T\_{\rm PCM}}{\partial y^2} \tag{1}
$$

<sup>∂</sup> *<sup>T</sup>*\_\_\_\_*air*

*cp*,*<sup>g</sup> mg*

*cp*,*air ρair Aair δair*

PVT direct heating

Normal air conditioning

*cp*

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> *uair*

**Table 2.** Operation models of the heat pump system with PVT collectors and PCMs.

<sup>∂</sup> *<sup>T</sup>*\_\_\_\_*<sup>g</sup>*,*<sup>i</sup>*

**Operation mode ON/OFF status of the dampers, fans and heat pump**

<sup>∂</sup> *<sup>T</sup>*\_\_\_\_\_*air*,*<sup>i</sup>*

**Figure 18.** Schematic of the PCM TES model.

\*\*The ON/OFF status is not related to the operation mode.

∂ *T*\_\_\_\_*air*

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> *Aair*(*α<sup>g</sup> <sup>I</sup>*

<sup>∂</sup>*<sup>x</sup>* <sup>+</sup> \_\_\_\_\_\_\_\_ <sup>1</sup> *ρair cp*,*air δair*

**D1 D2 D3 D4 D5 D6 D7 D8 PCM** 

OFF ON OFF OFF OFF ON ON OFF ON ON OFF

— — — OFF ON OFF OFF ON OFF — ON

\*If the PCM discharging mode can maintain the required indoor thermal comfort, the heat pump is switched off, damper D8 is OFF and damper D7 is ON. Otherwise, the heat pump will be used, damper D8 is ON and damper D7 is OFF.

PCM charging OFF ON ON OFF —\*\* OFF OFF — OFF ON — PVT exhausting ON OFF OFF — — — — — — ON — PCM discharging — OFF OFF ON OFF ON ON/OFF\* OFF/ON\* ON — OFF/ON\*

where *ρ* is the density, *h* is the specific enthalpy, *k* is the thermal conductivity, *u* is the velocity,

 is the specific heat capacity, *δ* is the thickness, *q* is the heat flux, *M* is the mass flow rate, *A* is the heat transfer area, *m* is the mass, the subscripts *PCM1* and *PCM2* represent the two PCM layers respectively, the subscripts *g*, *PV* and *b* represent the glass cover, the PV panel and

(*qPCM*1−*air* − *qair*−*PCM*2) (2)

**fan**

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

Solar-Assisted HVAC Systems with Integrated Phase Change Materials

**PVT fan**

**Heat pump**

35

*<sup>t</sup>* + *qnc*,*PV*−*g*,*<sup>i</sup>* + *qrad*,*PV*−*g*,*<sup>i</sup>* − *qwind* − *qsky*) (3)

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> *cp*,*air <sup>M</sup>*(*Tair*,*in*,*<sup>i</sup>* <sup>−</sup> *Tair*,*out*,*<sup>i</sup>*) <sup>+</sup> *Aair*(*qconv*,*p*−*air*,*<sup>i</sup>* <sup>+</sup> *qconv*,*b*−*air*,*<sup>i</sup>*) <sup>+</sup> <sup>2</sup> *<sup>A</sup>fin qconv*,*fin*−*air*,*<sup>i</sup>* (4)

**Figure 17.** Illustration of the system operating modes.


\*If the PCM discharging mode can maintain the required indoor thermal comfort, the heat pump is switched off, damper D8 is OFF and damper D7 is ON. Otherwise, the heat pump will be used, damper D8 is ON and damper D7 is OFF. \*\*The ON/OFF status is not related to the operation mode.

**Table 2.** Operation models of the heat pump system with PVT collectors and PCMs.

thermal comfort through opening the dampers D5 and D8. It should be noted that the PVT direct heating mode may cause overheating during the daytime. However, under some cold

The performance of this system was evaluated through numerical simulation. The heating demand of the house was simulated using TRNSYS, and the indoor thermostat setting was specified according to NatHERS [40]. A two-layer PCM TES model (**Figure 18**) and a PVT model developed in a previous study [41] were used to facilitate the performance simulation of the proposed system. The governing equations for the energy balance of the PCM layers and the fluid air in the PCM TES unit are described in Eqs. (1) and (2), respectively. In the model of the PVT collector, six nodes were vertically discretised, including the glass cover, PV plate, absorber plate, fins, fluid air and the bottom plate. The governing equations for the glass cover and the fluid air are described in Eqs. (3) and (4), respectively, while the governing

<sup>∂</sup> *<sup>h</sup>* \_\_\_\_\_ *PCM*

<sup>∂</sup>*<sup>t</sup>* <sup>=</sup> *kPCM*

<sup>∂</sup><sup>2</sup> *<sup>T</sup>* \_\_\_\_\_\_ *PCM*

<sup>∂</sup> *<sup>y</sup>*<sup>2</sup> (1)

weather conditions, the direct heating mode could be considered.

**Figure 16.** Schematic of the heat pump system with integrated PVT collectors and PCMs.

equations of the other nodes can be described in the same way.

*ρPCM*

34 Sustainable Air Conditioning Systems

**Figure 17.** Illustration of the system operating modes.

$$\frac{\partial \ T\_{\dot{w}}}{\partial t} = \mu\_{\dot{w}} \frac{\partial \ T\_{\dot{w}}}{\partial \mathbf{x}} + \frac{1}{\rho\_{\dot{w}} \frac{c}{c\_{\rho \dot{w} \dot{r}}} \frac{\partial}{\partial r}} (q\_{\text{PCM-air}} - q\_{\text{abs-PCM}}) \tag{2}$$

$$\sigma\_{p,\emptyset} m\_{\emptyset} \frac{\partial T\_{yj}}{\partial t} = A\_{ai} (\alpha\_{\emptyset} I\_t + q\_{ncPV-yj} + q\_{ndPV-yj} - q\_{wind} - q\_{sky}) \tag{3}$$

$$\mathcal{L}\_{p, \text{air}} \rho\_{\text{air}} A\_{\text{air}} \delta\_{\text{air}} \frac{\partial T\_{\text{air}j}}{\partial t} = \mathcal{L}\_{p, \text{air}} M(T\_{\text{air}, \text{air}} - T\_{\text{air}, \text{at}, \text{l}}) + A\_{\text{ul}} (q\_{\text{conv, p-air}, \text{l}} + q\_{\text{conv, b-air}, \text{l}}) + 2 \, A\_{\text{fin}} q\_{\text{conv, fu-in}, \text{l}} \tag{4}$$

where *ρ* is the density, *h* is the specific enthalpy, *k* is the thermal conductivity, *u* is the velocity, *cp* is the specific heat capacity, *δ* is the thickness, *q* is the heat flux, *M* is the mass flow rate, *A* is the heat transfer area, *m* is the mass, the subscripts *PCM1* and *PCM2* represent the two PCM layers respectively, the subscripts *g*, *PV* and *b* represent the glass cover, the PV panel and

**Figure 18.** Schematic of the PCM TES model.

the bottom plate of the PVT collectors, and the subscripts *conv*, *rad*, *nc*, *wind* and *sky* indicate convective, radiative, natural convective, wind-driven and sky radiative, respectively. More details about the PVT model and the model validation can be found in Ref. [41].

The coefficient of performance (COP) of the heat pump system with the integrated PVT collectors and PCMs is determined by Eq. (5).

\*\*C.\*\*C.\*\* 2 uma 1 \*\*C.\*\* us is \*\*c.\*\* uma \*\*c.\*\*  $\text{buj. (c): }$ 

$$\text{COP = } \frac{Q\_{\text{today}}}{E\_{\text{tpp}} + E\_{\text{p7T}\text{fm}} + E\_{\text{pCM}\text{fm}}} . \tag{5}$$

**5. Conclusion**

Electricity (kWh)

can always operate at the optimal performance.

Solar-assisted HVAC system with integrated PCMs are a good alternative to conventional fossil fuel-driven vapour compression systems. Due to the intermittency of solar energy, the integration of PCM thermal energy storage units with solar-assisted HVAC systems provides a great opportunity to maximise the utilisation of solar energy and thus to increase the efficiency of HVAC systems. Two different HVAC systems with integrated PCMs and solar photovoltaic thermal collectors were presented, and their performance was investigated. The results showed that the solar thermal contribution for the desiccant wheel regeneration was 96.5% when using integrated PVT collectors and PCMs during the test period. The average COP of the heat pump system with integrated PVT collectors and PCMs for space heating was 5.21 during the test period, which was higher than the baseline case with a COP of 3.06. The results showed that the system performance can be improved through the integration of PCMs. For solar-assisted HVAC systems, control will be essential to ensure that the system

**Figure 21.** ON/OFF state of the heat pump under the PVT-PCM case and the *baseline* case.

<sup>0</sup> <sup>12</sup> <sup>24</sup> <sup>36</sup> <sup>48</sup> <sup>60</sup> <sup>72</sup> <sup>84</sup> <sup>96</sup> <sup>108</sup> <sup>120</sup> <sup>132</sup> <sup>144</sup> <sup>156</sup> <sup>168</sup> -100

**Figure 20.** Accumulated electricity generation and consumption under the *PVT-PCM* case and the *baseline case*.

PVT-PCM case, Net electricity generation PVT-PCM case, Electricity generation PVT-PCM case, Total electricity consumption Baseline case, Total electricity consumption

Time (h)

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37

where *E* is the electrical energy consumed, and *Qheating* is the heating energy demand.

To evaluate the performance of this system, two cases were designed and simulated under winter weather conditions in Melbourne. In the *baseline* case, only an air source heat pump was used to condition an Australian house. In the *PVT-PCM* case, the air source heat pump integrated with PVT and PCMs was used for space heating. **Figure 19** presents the one-week winter weather conditions used for performance tests.

**Figure 20** compares the accumulated electricity generation and consumption during the test days and **Figure 21** illustrates the operation status of the heat pump unit under the two test cases. It is worthwhile to note that the PVT direct heating mode was not considered in this study. It can be seen that the accumulated electricity consumption under the PVT-PCM case was 42.3 kWh, which was much lower than the baseline case of 72.0 kWh. In comparison to the baseline case, the electricity saving was 41.1%, which was achieved through decreasing the operating time of the air source heat pump. The total electricity generation of the PV panels was 156.5 kWh under the *PVT-PCM* case. The average COP of the heat pump system with the integrated PCM layers in the building ceiling and PVT collectors for space heating during the selected week was 5.21, which was higher than that of the air source heat pump system in the *baseline* case (i.e. 3.06). The above-mentioned results indicated that this integrated heat pump system with PCMs and PVT collectors can substantially reduce the electricity consumption for winter space heating.

**Figure 19.** Weather data of a winter week.

**Figure 20.** Accumulated electricity generation and consumption under the *PVT-PCM* case and the *baseline case*.

**Figure 21.** ON/OFF state of the heat pump under the PVT-PCM case and the *baseline* case.
