Feature Article

Energy Modeling For Sustainability

April 1, 2008

Chief among factors determining the sustainable performance of a building are the amount of energy it consumes and how much of that energy is from renewable sources. Architects have long relied on engineers to perform the complex energy consumption estimations and select mechanical systems. This reliance is now transforming into collaboration as engineers are being invited into early design phases to help the team with decisions about orientation and massing. Three case studies show how the engineers’ work with energy simulation software informs those decisions. For many years, the U.S. Department of Energy (DOE) funded the development of the software now known as DOE-2. It analyzes the reaction of a building’s skin and geometry in response to internal loads and external climate conditions for each of the 8,760 hours in a year. These loads include solar heat gains; heat gains from occupants, electric lights, and equipment; and heat gained or lost through infiltration or by conduction through the walls, roof, and glazing. DOE-2 also models mechanical system performance and the local utility rate structure. The Department of Energy is now promoting the next-generation EnergyPlus. Like its predecessor DOE-2, EnergyPlus depends on third-party software to provide a user-friendly graphical interface.

Simulation software supplies a nuanced view of a building’s dynamic thermal performance that enables engineers to select and size equipment that is smarter, smaller, and often less expensive than the norm. For instance, cold but clear winter days may enjoy abundant solar radiation. Peak cooling loads may occur at a time of maximum daylight, which can replace some or all of the electric lighting. Economizers can use relatively cool outdoor air in lieu of some mechanical cooling. Looking at all these factors and their complex interactions is known as “whole-building simulation.” Optimizing building performance can often result in significant construction cost savings due to smaller systems and mechanical rooms.