This thesis investigates air-based building integrated photovoltaic/thermal (BIPV/T) systems. A building-integrated photovoltaic/thermal (BIPV/T) system converts solar energy into electricity and useful heat, while also serving as the functional exterior layer of the building envelope and thereby achieving the design of net-zero energy buildings. A comprehensive literature survey of a variety of BIPV/T systems points out the need to develop enhanced air-based BIPV/T systems with aesthetical and mechanical requirements taken into account. This thesis examines improved designs of open-loop air-based BIPV/T systems both numerically and experimentally. A BIPV/T design with two inlets was proposed and a prototype using custom-made frameless PV modules was constructed for feasibility validation. The experiments were performed using a solar irradiance simulator and included testing under varying irradiance levels, flow rates and wind speeds. Experimental results validated that the two-inlet BIPV/T concept improved thermal efficiency by 5% compared to a conventional single-inlet system. Detailed BIPV/T channel air temperature measurements showed that the mixing of the warm outlet air from the first section and the cool ambient air drawn in from the second inlet contributes to the improved performance of the two-inlet system. The heat transfer characteristics in the BIPV/T channel between air and PV panel was studied through the development of Nusselt number correlations. Comparative tests were also conducted on a prototype using opaque mono-crystalline PV modules and a prototype using semi-transparent mono-crystalline PV modules. Results showed that applying semi-transparent PV modules (with 80% module area covered by solar cells) in BIPV/T systems increased thermal efficiency (ratio between the thermal energy recovered by the channel air and solar energy incident on the upper surface of PV) by up to 7.6% compared to opaque ones, particularly when combined with multiple inlets. A variation of this two-inlet BIPV/T design that includes a vertical solar air heater embedded with a packing material (wire mesh) was presented and analyzed. The additional solar air heater receives high amount of solar energy during the winter period when solar altitude is low, enabling the outlet air to be heated to a higher temperature. A lumped parameter thermal network model of this BIPV/T system was verified using experimental data obtained for a single-inlet BIPV/T prototype. Simulation results indicate that the application of two inlets on a BIPV/T collector increases thermal efficiency by about 5% and increases electrical efficiency marginally. An added vertical glazed solar air collector improves the thermal efficiency by about 8%, and the improvement is more significant with wire mesh packing in the collector by an increase of about 10%. A case study is performed using this lumped thermal model and showed that the thermal efficiency of a BIPV/T roof of an existing solar house is improved by 7% with four air inlets. In conclusion, this thesis presents validated models for the design of open-loop BIPV/T air systems with multiple inlets and possibly semi-transparent PV covers.