Study on expansion of air-conditioning area by a hybrid ground source heat pump system for retrofitting an office building to be a net-zero energy building

Abstract

In recent years, ground source heat pump (GSHP) systems have gained recognition as a renewable heat energy utilization application and are increasingly installed in buildings to achieve Zero Energy Building status in Japan. In Kitakyushu City, Japan, an office building operational since 2010 features a hybrid GSHP system comprising 80 m x 50 borehole double U-tube ground heat exchangers and a cooling tower. Monitoring of this hybrid GSHP system over more than 8 years revealed an estimated effective thermal conductivity of the ground at 4.5 W/(mK), higher than thermal response test results due to groundwater flow influence, with the maximum cooling load smaller than the design value.||In this paper, the authors aimed to expand the air-conditioning area using the hybrid GSHP system to retrofit the office building into a net-zero energy building. Specifically, simulation was employed to investigate the air-conditioning area covered by the hybrid GSHP system, based on actual measurements of heating and cooling loads. Results confirmed a threefold increase in the air-conditioning area covered by the hybrid GSHP system, expanding from approximately 20% to 60% of the present air-conditioning area. Comparatively, when compared to an all-air-source heat pump (ASHP) scenario, electric energy reduction increased with building area, showing approximately 13% reduction when doubled and approximately 19% increase when tripled.