Investigation of Phase Change Materials on Australian Residential Building Energy Efficiency

Abstract

This paper investigates the effectiveness of Phase Change Materials (PCMs) in building envelopes on the energy efficiency of Australian residential buildings. Using DesignBuilder simulation, a representative Australian single storey residential house was modelled with different PCM application strategies under a range of Australian climates. In this case study, PCMs could achieve 2.3–16.3% annual electricity savings depending on types of PCMs and climates except in Australian Climate Zone 1 (Darwin). Simulation results also indicated that applying PCMs on external walls would improve energy efficiency performance more than applying them to the ceiling, and PCMs on longer solar exposed walls performed better than those applied on shorter solar exposed facades. It was also found that the energy efficiency performance would decrease when the PCM melting point was outside the thermostat range of the particular climate.

Appendix

Details of PCM modelling in DesignBuilder

The calculation method for the fully implicit scheme inside a homogeneous material is described by:

$$C_{p} \rho \Delta x\frac{{T_{i}^{j + 1} - T_{i}^{j} }}{\Delta t} = k_{w} \frac{{T_{i + 1}^{j + 1} - T_{i}^{j + 1} }}{\Delta x} + k_{E} \frac{{T_{i - 1}^{j + 1} - T_{i}^{j + 1} }}{\Delta x}$$

(62.1)

For Eq. (62.1), $T$ is the node temperature, $C_{p}$ is specific heat, $\rho$ is density, $\Delta x$ is finite difference layer thickness defining distance between nodes, and $\Delta t$ is simulation time step. The subscripts refer to the time step: $i$ is the current node while $i - 1$ and $i + 1$ are the adjacent exterior and interior nodes of the construction respectively, and $j + 1$ and $j$ are the next and previous time steps, respectively. All elements are discretized using Eq. (62.2), which depends on a space discretization constant $c$, the thermal diffusivity of the material $\alpha$, the time step $\Delta t$, and Fourier number $F_{0}$.

$$\Delta x = \sqrt {c\alpha \Delta t} = \sqrt {\frac{\alpha \Delta t}{{F_{0} }}}$$

(62.2)

Phase change materials have temperature dependent specific heat capacity $C_{p}$ which is updated at each time step according to Eq. (62.3) to ensure that the correct enthalpy and $C_{p}$ are used in every time step.

$$C_{p} = \frac{{h_{i}^{j} - h_{i}^{j - 1} }}{{T_{i}^{j} - T_{i}^{j - 1} }}$$

(62.3)

where $h$ is enthalpy of material. DesignBuilder requires temperature dependent thermo-physical properties for PCM materials to account for enthalpy changes during phase change. This study used the commercially available Bio-PCM, from the DesignBuilder library. Figure 62.4 shows the enthalpy-temperature curve of the Bio-PCM with a melting temperature of 23 °C. Each PCM used has a melting temperature range of 4 °C.

Fig. 62.4

Enthalpy–temperature curve of Bio-PCM23 (DesignBuilder 2019)

Fig. 62.5

Internal load schedules (DesignBuilder 2019)