Buildings consume 40% of primary energy in the U.S. This seminar introduces our research in simulation tools for low energy buildings. The goal is to reduce energy consumption and peak power demand while maintaining or improving the indoor environment quality. To quickly provide spatial and temporal airflow information for ventilation system design and indoor environment control, we have applied and further developed a fast fluid dynamics (FFD) model, which is 50 times faster than computational fluid dynamics (CFD). By running the FFD simulation in parallel on a graphics processing unit, it can be 1500 times faster than CFD on a CPU. To enable rapid prototyping of new HVAC systems and to support the design and performance assessment of advanced control systems, we have developed an open-source Modelica library for building energy and control systems. By separating physical modeling from numerical solution, our approach can make modeling flexible and fast to accelerate innovation of low energy systems. In addition, the coupling of FFD on GPU and Modelica will be useful for the design and control of HVAC system for indoor environment with non-uniform air distribution, such as auditoriums, data centers and buildings using natural ventilation.