Towards Zero-Energy Buildings: A Protocol for Calculating Building Loads

Photo of Chris Pettit, Thomas Saller, Hunter Wright Students: Chris Pettit, Thomas Saller, Hunter Wright

Sponsor: The University of Texas at Austin

Date: Fall 2009

Requirements:
The model must account for transient effects. In addition, it must apply to cities in Texas, and be accurate to within 10% of actual loads. Once a computer model has been developed, a sensitivity analysis should be performed to determine the effects of at least seven building characteristics on thermal load. Furthermore, three case studies should be run with an accurate thermal load calculator to determine whether they have been oversized. An online building load calculator will be created to provide a free resource to Texas homeowners.

Problem:
Energy conservation is a growing global concern. Texans are the largest consumers of energy per capita in the nation. A large portion of this energy is used to cool our buildings. Current standards for sizing HVAC systems use peak design conditions that do not account for transient building effects. This leads to the installation of oversized, less efficient systems that cost more to maintain and waste energy. Our primary objective is to create a transient building load calculator in MATLAB.

Solution:
The team created an energy balance model using MATLAB. The code accounted for eight modes of heat transfer: conduction, convection, solar radiation, blackbody radiation, infiltration, thermal capacitance, internal generation from lighting and equipment, and the heat removed or added by the HVAC system. For one case study, the program calculated a maximum thermal load of 15,800 BTU per hour, almost one half of what the example building was sized for. The sensitivity analysis showed that insulation and thermal capacitance have larger effects than temperature set points, infiltration, and heat generation. Because of the inaccuracy of the MATLAB model, the design team also created an online web calculator using standard HVAC methods, which can be viewed at http://www.bload.info.

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