**2.1. Photovoltaic-thermal collectors**

According to the heat extraction methods used, PVT collectors can be categorised into airbased PVT, liquid-based PVT, heat pipe-based PVT, PCM-based PVT and thermoelectric-based PVT [7], among which air-based PVT and liquid-based PVT have been commonly studied. The research on PVT collectors has been primarily focussing on the model development and performance analysis, collector design and optimisation, and the integration of PVT collectors with building HVAC systems. **Figure 1** presents an air-based PVT collector, which consists of a PV panel, an absorber plate, a bottom plate and an insulation layer. An air channel is created between the absorber plate and the bottom plate to allow the air flowing through to take away the heat to increase the efficiency of electricity generation. The heated air can be directly used for space heating or to drive desiccant cooling systems. **Figure 2** provides the air-based PVT collectors implemented in a net-zero energy office building, in which the heated air from the PVT collectors was directed into the building, in winter, for space heating.

**Figure 1.** Illustration of an air-based PVT collector.

collectors and thermal energy storage (TES) using phase change materials (PCMs) are receiv-

Solar PVT collector is a combination of photovoltaics and solar thermal systems, which can generate both electricity and thermal energy simultaneously from one integrated component [1]. Compared to the use of separate solar technologies for heat and electricity generation, using solar PVT collectors is more cost-effective [2]. Phase change material (PCM) with a high energy storage density and the capacity to store thermal energy at a relatively constant temperature is being considered as a future technology to reduce building operating cost and provide better indoor thermal comfort [3, 4]. PCM has been used to increase the local thermal mass of building envelopes or incorporated into building HVAC systems to provide functional purposes and enhance overall system efficiency. As solar energy is intermittent and has a low energy density, the integration of PCMs with PVT collectors may provide an alternative solution to effectively use solar energy and enhance the performance of building HVAC systems. For instance, Su et al. [5] investigated the performance of an air-based PVT collector integrated with PCMs using different configurations. The results indicated that by attaching a PCM layer on the upper side of the air channel of the PVT collector, the overall efficiency of the PVT collector can be improved by 10.7% in comparison to the use of the same PVT without using the PCM. The electrical and thermal performance of a water-based PVT collector incorporating PCMs was also studied [6]. The results showed that the energy output of the PVT collector can be maximised by incorporating a PCM layer with a thickness of 3.4 cm and

ing increasing attention for developing high-performance HVAC systems.

a melting point of 40°C under the weather conditions in Nanjing, China.

PVT collectors and PCMs. Section 5 provides a summary of this chapter.

PVT collectors was directed into the building, in winter, for space heating.

**materials**

22 Sustainable Air Conditioning Systems

**2.1. Photovoltaic-thermal collectors**

This chapter is structured as follows. Section 2 presents a brief introduction to solar PVT collectors and PCMs. Section 3 provides an overview of solar-assisted HVAC systems with integrated PCMs. Section 4 presents the development of two HVAC systems with integrated

**2. Introduction to photovoltaic-thermal collectors and phase change** 

According to the heat extraction methods used, PVT collectors can be categorised into airbased PVT, liquid-based PVT, heat pipe-based PVT, PCM-based PVT and thermoelectric-based PVT [7], among which air-based PVT and liquid-based PVT have been commonly studied. The research on PVT collectors has been primarily focussing on the model development and performance analysis, collector design and optimisation, and the integration of PVT collectors with building HVAC systems. **Figure 1** presents an air-based PVT collector, which consists of a PV panel, an absorber plate, a bottom plate and an insulation layer. An air channel is created between the absorber plate and the bottom plate to allow the air flowing through to take away the heat to increase the efficiency of electricity generation. The heated air can be directly used for space heating or to drive desiccant cooling systems. **Figure 2** provides the air-based PVT collectors implemented in a net-zero energy office building, in which the heated air from the The performance of air-based PVT collectors is highly influenced by the factors such as solar irradiation, air flow rate, the slope and the orientation of the collectors. **Figure 3** shows the measured solar irradiance, ambient air temperature, the inlet air temperature and outlet air temperature of a PVT collector tested in a laboratory-scale test rig (**Figure 4**) under the two summer test days in 2014 with a fan operating frequency of 10 Hz (i.e. ~50 L/s). It can be seen that, during the daytime, a maximum temperature rise of 14.0°C was achieved when the air flowed through the PVT collectors. During the night-time, a reduction in the air temperature of 1–2°C was achieved in the two test days through night-time radiative cooling. **Figure 5** illustrates the instantaneous thermal energy collected, electricity generated and the electrical efficiency of the PVT collectors under the two summer test days. There was a slight delay between the electricity generation and thermal energy generation due to the low response of temperature to the changes in solar radiation. The maximum instantaneous thermal energy collected from the PVT collectors and the maximum instantaneous electricity generation in the two summer test days were 840 W and 408 W, respectively. The electrical efficiency of the PV cells was relatively stable except during the morning of the first test day due to large variations in solar irradiance.

The above-mentioned results showed that, besides electricity generation, a substantial amount of thermal energy can also be collected. The thermal energy collected during the winter daytime can be directly used for space heating, and the thermal energy collected during the summer night-time can be directly used for space cooling. However, the thermal energy collected during the summer daytime cannot be directly used for space conditioning unless it is used to drive air conditioning systems such as rotary desiccant cooling systems.

**Figure 2.** Air-based PVT collectors implemented in a net-zero energy office building.


0 4 8 12 16 20 24 28 32 36 40 44 48

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

Electricity generated Thermal energy collected Electrical efficiency

Time (hour)


summer test days.

0

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

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

Energy (W)

1000

0 5

Solar-Assisted HVAC Systems with Integrated Phase Change Materials

Electrical efficiency (%)

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

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