Sensitivity of Building Energy Demand to Solar Irradiance

Buildings react to their meteorological environment and surrounding microclimate in specific ways, depending on the season or climate regime, the orientation of the façade, and the material properties of the envelope systems. Because a bill is not paid to the Sun for the energy supplied to the building, the incident solar irradiance is often neglected by the occupants, building managers, and design teams, despite being the primary energetic driver. Further complicating the issue, the statistical differences between measured and modeled solar irradiance on tilted surfaces are significant. Meteorological data is used during the design phase for building energy modeling to determine the benefits, costs, and inter-system impacts of design configurations. However, meteorological data also affects control decisions in buildings after commissioning, which in turn affects whole building energy balances, building zone comfort violations, and financial obligations of building owners or tenants. New strategies in advanced controls for building energy management systems have distinct limitations when the quality of information from the solar resource is not descriptive of the actual irradiance coupling energy fluxes between the interior zones. The the US Department of Energy has identified that on-site meteorological sensing has limited and insufficient implementation with buildings systems. Solar and microclimate information networked to advanced controls in buildings presents the opportunity for a 15-30% energy savings.1,2 This presents a clear missed opportunity to understand the interaction between building components (window systems, walls, roofing, HVAC, etc.) and their synergistic responses to the whole building envelope energy flux. Following Lord Kelvin, “If you cannot measure it, you cannot improve it”, and this study references modeled data against measured plane of array (POA) values on a commercial building in the mid-Atlantic region. Inverse modeling of a mid-sized commercial building in the Philadelphia Navy Yard was conducted to derive sensitivity responses for factors of outdoor air temperature, irradiance through windows, and irradiance absorbed by opaque walls. Analyses demonstrated that although irradiance data for vertical surfaces has a significantly lower sensitivity compared to that of outdoor air temperature, changing the input data from measured POA values to those generated by empirical modeling of commercially available Global Horizontal Irradiance (GHI) does influence energy loads by 5 to 20%, seasonally.

1) M.R. Brambley, D. Hansen, P. Haves, D.R. Holmberg, S.C. McDonald, K.W. Roth, and P. Torcellini, Advanced Sensors and Controls for Building Applications: Market Assessment and Potential R&D Pathways, Prepared for the U.S. Department of Energy, April 2005, p. 37.

2) V. M. Zavala. Proactive Energy Management for High-Performance Buildings: Exploiting and Motivating Sensor Technologies. Future of Instrumentation International Workshop, 2011