Session: 03-01: Advances in Indoor Environment Technologies and Solutions

Paper Number: 155053

155053 - Similitude for Buildings Conditioned Exclusively by Ambient Energy 

Abstract: 

Ambient energy (from the sun, air, sky and ground) is abundant and ubiquitous, and provides an alternative for many low-temperature applications, particularly space conditioning. Passive solar heating and nighttime ventilation cooling can entirely serve heating and cooling needs across US climates. Sky radiation has greater cooling potential and reduces thermal mass required for 100% ambient cooling. Ambient conditioning would largely eliminate seasonal variation in demand and reduce required grid generation and transmission capacity, making the renewable transition more feasible and economical.

To facilitate adoption of ambient conditioning, design tools and broad design expertise are needed. Several whole-building simulations are capable of detailed energy performance predictions, but skill and time requirements constitute barriers to widespread use. While commercial buildings with larger budgets are frequently simulated, residential buildings, which comprise about 90% of the market, are seldom studied in such detail. A simple, accessible tool that distills energy balance principles to a few universal parameters could stimulate the growth of an ambient-conditioning industry, and as a result, reduce climate change.

Key variables necessary to model ambient-conditioned buildings are envelope loss, internal heat gain, solar gain, ambient cooling loss, and thermal capacitance. Buckingham Pi reduces these five primitive variables to two similitude parameters - a ratio of gains over losses, and a thermal time constant T (thermal capacitance/envelope loss). However, greater insight can arguably be gained by separating the first parameter into three parts, solar load ratio SLR (solar gain/envelope loss), internal heat load ratio ILR (internal gain/envelope loss), and ventilation load ratio VLR (ventilation cooling/envelope loss, or similar sky load ratio for sky cooling). 

One simulation with particular weather data determines the performance of a wide range of buildings with the same parameter values. Further, these parameters have quantitative thresholds to achieve 100% ambient conditioning. Specifically, ILR + SLR averaged over time of order T must be 1 or larger to avoid decreasing indoor temperature. VLR > ILR is needed to avoid overheating. Finally, T should be comparable to the longest interval of unavailability of solar gains and of ambient cooling potential. These thresholds provide objectives for building design before a floor plan or other details are considered, and if followed throughout the entire design process, ensure performance of the final product. 

Correspondence of similitude parameter values to building performance is demonstrated with measured indoor temperature and archived weather data for 2022 for an ambient-conditioned home near Pagosa Springs, CO (a heating-dominated climate). During the heating season, ILR varied from 0.35 to 0.56, while SLR was 0.28 – 0.95. The lowest weekly average ILR + SLR was 0.77 in early December, followed by weekly averages of 0.90, 1.1, and 0.97. Indoor temperature followed this trend, decreasing significantly during the first two weeks, recovering slightly during the third week, and reached its lowest value of the year of 17.5 °C (63.5 °F) during the fourth week. Thermal mass (T of 2.2 – 3.1 days) was sufficient to keep indoor temperature within the comfort range during most of this period, as well as for weeks with ILR + SLR = 0.93 and 0.83 in late February/early March separated by a sunny week with ILR + SLR = 1.40.

In this climate, internal heat gains dominate the cooling demand, with ILR ranging from 1.7 to 7.2 during the cooling season. VLR ranged from 4.5 to 49.6, and was always larger than ILR, indicating that ventilation cooling is consistently capable of entirely serving the cooling load.

These results support the usefulness of similitude for designing ambient-conditioned buildings.

Presenting Author: M. Keith Sharp Univ Of Louisville

Presenting Author Biography: M. Keith Sharp, Emeritus Professor, ASME Fellow, PE, Department of Mechanical Engineering, University of Louisville (UL), received a BS from the University of Cincinnati in 1976, MS from Colorado State University (CSU) in 1978 and ScD from the Massachusetts Institute of Technology (MIT) in 1987, all in Mechanical Engineering. He was Director of the Renewable Energy Applications Laboratory at UL, and is a past chair of the Solar Energy Division of ASME. He studied stratification in solar thermal storage at CSU. Current research includes heating and cooling of buildings entirely by ambient sources of energy. He has published articles in Solar Energy, the Journal of Ambient Energy, the Journal of Solar Energy Engineering, the Journal of Engineering for Sustainable Buildings and Cities, the International Journal of Sustainable Energy, the Journal of Energy, the Journal of Energy Engineering, and Solar Today. He has designed and built a number of ambient-conditioned homes. His current home is entirely heated by passive solar, except for occasional fires in a fireplace, and entirely cooled by nighttime ventilation. He has developed software for simulating ambient-conditioned buildings that is easy to use for architects and builders during the predesign phase. Such early energy analysis helps ensure performance of the final building.