For the purpose of utilizing the cooling effect and solar shading of green roof plants, a system combining the outdoor unit of air-conditioners with green roof plants is proposed. This paper describes the method of predicting the energy savings of the proposed system. An improved leaf transpiration model is developed to estimate how much water can be evaporated by the plant transpiration, and a leaf boundary model is newly developed to estimate how much air temperature can be decreased by the plant transpiration. An experimental system was built using hydroponic-cultivated sweet potato. The solar shading and transpiration-cooling effects are measured and used to verify the accuracy of two newly developed models. The average and root mean square error (RMSE) are 2.9% and 18.8% for transpiration estimation, and 3.3% and 94.0% for estimating air temperature decrease. Further, the specification of six main air-conditioner manufactures are analyzed and divided into to three groups of high, middle and low improvement of energy efficiency accompanying to the outdoor air temperature decrease. Then the cooling loads of a standard office building at four typical climate area of Sapporo, Tokyo, Osaka, and Naha are simulated and used to estimate the energy consumed by the three groups of air-conditioner at the conditions of with and without combining to hydroponic-cultivated roof plant. The results show that the energy saving rates are similar at four different areas, which are about 8% for the high efficiency-improvement group, 6% for middle efficiency-improvement group and 1% for low efficiency-improvement group.
