Optimizing phase change material integration in residential building envelopes for year-round energy efficiency in cold climates

Phase Change Materials (PCMs) hold significant potential for improving traditional building envelopes by mitigating indoor temperature fluctuations and reducing energy demands through their Thermal Energy Storage (TES) properties. A crucial objective in designing PCM-enhanced building envelopes is to optimize their energy-saving performance under varying conditions. This simulation study focuses on a residential building in Alberta, Canada, analyzing both steady (normal) and intermittent (night) operation schedules. The aim is to identify PCM specifications that maximize year-round energy-saving. The two main variables of PCM specifications investigated are the midpoint melting temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mid</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> and the installation position (PCM layer on the interior or exterior side of the insulation layer). Preliminary simulations show that PCMs installed in the interior outperform those installed in exterior locations. If true, the optimization problem is simplified to a one-dimensional model, with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mid</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> being the continuous variable optimized to minimize cooling, heating, and total energy demand on an annual basis, respectively. The co-simulation of EnergyPlus and GenOpt platforms is employed for optimization. Results indicate that the PCM configuration with the optimal midpoint melting temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mid</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> resulted in total energy-saving of 6.65% at 21.71°C for the steady schedule and 5.21% at 22.04°C for the intermittent schedule. And the ratio of energy-saving for cooling was higher under intermittent operation (28.42% at 23.27°C) than under steady operation (22.38% at 22.76°C). Relatively satisfactory heating or cooling energy-saving was achieved when <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>mid</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> was set within ±0.5°C of the heating setpoint or 1°C–2°C below the cooling setpoint, respectively. For residential buildings in cold climates, the melting heat of PCMs primarily originates from indoor sources rather than outdoors. While the energy savings from PCMs during the winter are modest, their ability to mitigate indoor temperature fluctuations is significantly enhanced under intermittent operations, showing promise in enhancing thermal comfort and improving building energy flexibility.