Operational Performance of a Small-Scale NZEB Office and Effective Utilization of Photovoltaic Surplus Power on Non-Business Days using Base Air-Conditioning

Net-Zero Energy Buildings (NZEBs) play a key role in achieving carbon neutrality in the commercial sector, and they are being promoted actively across many countries through development of comprehensive roadmaps, guidelines, and supportive policies. However, in some regions of Japan, contractual restrictions on the reverse power flow of surplus electricity generated by photovoltaics (PV) limit the effective utilization of on-site renewable energy in NZEBs. Despite the growing deployment of NZEBs, limited studies have examined operational strategies that enhance on-site consumption of surplus PV electricity under such restrictions. Therefore, the objective of this study is to evaluate the operational performance of NZEB technologies and to examine a practical strategy for utilizing surplus PV electricity in a small-scale office building located in a warm climate region of Japan. The target building achieved NZEB status at the design stage and is equipped with an air-conditioning system that utilizes groundwater-based cooling and radiant cooling. A field investigation was conducted during summer, including a trial operation of "Holiday Air-Conditioning Mode," in which base air conditioning is operated on non-business days to absorb surplus PV electricity and reduce the thermal storage load at the beginning of the following workweek. The results indicate that Holiday Air-Conditioning Mode reduced the electricity consumption of the packaged air conditioner serving the main second-floor office on Monday and improved the indoor thermal environment. These improvements are attributed to the reduction of accumulated thermal load within the building structure through pre-cooling during holidays. As a complementary operational measure to further enhance the effective utilization of surplus PV electricity, changing the weekday radiant cooling operation from radiant panels to radiant floor cooling reduced the total electricity consumption by approximately 20% and improved the system coefficient of performance by about 0.5. These findings demonstrate that operational strategies utilizing building thermal mass can effectively mitigate PV curtailment and enhance the energy performance of NZEBs under reverse power flow restrictions