Demand response flexibility with synergies on passive PCM walls, BIPVs, and active air-conditioning system in a subtropical climate

Energy-efficient buildings and renewable power supply are two mainstreams for achieving zero-energy buildings and contributing to the energy paradigm transition towards carbon neutrality from building sectors. Due to the significant heat loss and large surface areas, building envelopes are full of potentials in reducing the cooling/heating load through enhanced thermal insulation performance and converting solar radiation into renewable energy with photovoltaics (PVs). However, smart connection with synergic complementary functions has not been comprehensively studied among heating, ventilation, and air-conditioning (HVAC) systems, phase change material (PCM) wall, and building integrated PVs (BIPVs). In this study, a transient and dynamic platform for energy-efficient buildings was established, integrating PCM walls and BIPVs. Multiple functions of the platform can be achieved, e.g., enhancement in thermal inertia and storage capacity of building envelopes with PCM, solar shading from BIPVs, renewable energy-driven HVAC, thermal regeneration on PCM with nighttime cooling and pre-cooling in the built environment. Synergistic functions between BIPVs and passive PCM layer are studied, e.g., the BIPVs can provide solar shading and thermally regenerate the PCM to resist the exterior heat flux during 1st-7th Aug. In-depth analysis was conducted on thermodynamics of PCMs, in respect to different geometrical (e.g., PCM thickness) and thermophysical parameters (e.g., latent heat and thermal conductivity on PCM). Afterwards, flexible charging/discharging strategies on latent heat with smart control on an indoor setpoint of HVAC were proposed, implemented, and compared, so as to shift grid electricity from off-peak to peak period. The proposed PCM charging/discharging strategies include pre-cooling for load shifting and floating indoor air setpoint. Parametrical analysis on different off-peak electric prices was conducted, to provide qualitative results on annual import cost savings. Research results indicate that, due to the shading of solar radiation, the vertical BIPVs will reduce the building cooling load from 73.5 to 67.9 kWh/m(2).a by 7.6%, and the peak power from 503.1 to 466.8 kW by 7.2%. Synergistic functions between BIPVs and passive PCM walls can resist the heat flux with a higher magnitude for a shorter period during 1st-7th Aug, and provide heat with a higher magnitude for a longer period during 1st-7th Jan. Furthermore, by adopting the pre-cooling strategy with smart operation on HVAC systems for PCM regeneration, the PCM melting fraction range can be expanded from [0.92, 1] to [0.75, 1], together with the increase in accumulated heat from 307427.8 to 337881.1 kJ by 9.9%. By adopting the floating indoor air setpoint temperature strategy with smart operation on HVAC systems, the annual import cost is decreased from 65.1 to 61.1 HK$/m(2).a by 6.1%. Economic performance analysis indicates that, with the grid feed-in tariff in Hong Kong, the simple payback time (SPBT) is around 17 years, while the PCM layer will prolong the SPBT to exceed 20 years. This study can enhance the thermal regulation capacity of building envelopes, together with a flexible connection between active HAVC systems and passive PCM walls with techno-economic viability.