PREDICTIVE HEAT AND MASS TRANSFER MODEL OF PLANT-BASED ROOFING MATERIALS FOR ASSESSMENT OF ENERGY SAVINGS

Open Access
Author:
Tabares Velasco, Paulo Cesar
Graduate Program:
Architectural Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 23, 2009
Committee Members:
  • Jelena Srebric, Dissertation Advisor/Co-Advisor
  • Jelena Srebric, Committee Chair/Co-Chair
  • Bohumil Kasal, Committee Member
  • Stanley Allan Mumma, Committee Member
  • Robert Berghage Jr., Committee Member
Keywords:
  • roof heat fluxes
  • heat and mass transfer
  • building energy
  • evapotranspiration
  • green roofs
  • vegetated roof
Abstract:
Green roofs are becoming popular in the U.S. with the green roof industry growing at a rate of 30-50% from 2001 to 2008. Green roofs are a sustainable technology that could potentially offer several benefits to society and the environment. There have been several models proposing different ways to represent models of green roof systems. Until now, none of these models have been properly verified and validated. Moreover, to the best of our knowledge, there is no single study that has measured all of the important heat and mass transfer processes simultaneously. Thus, the overall objective of this thesis is to develop a predictive heat and mass transfer model for green roof systems in summer conditions. The model is also verified and validated with experimental data from the “Cold Plate,” an experimental apparatus specifically designed and built to quantify heat and mass transfer processes. The “Cold Plate” apparatus represents a new kind of apparatus that addresses the shortcomings in the existing data sets on energy balance for green roofs. Experiments were conducted in a full-scale environmental chamber that simulated outdoor conditions. Currently, there is no other experimental apparatus that simultaneously measures the same physical phenomena. Overall, more than 10 experiments were conducted inside the environmental chamber. Evapotranspiration had the role of controlling the intensity of all other heat fluxes by modulating or diverting incoming and outgoing heat fluxes, depending on the state of the plants and environmental conditions. Interestingly, the lowest conductive heat fluxes through the green roof were consistently found when the green roof was the wettest. This finding also addresses the old dilemma regarding the tradeoffs between having a dry or a wet green roof. A new green roof model is proposed. The model considers heat and mass transfer processes between the sky, plants, and substrate. Based on laboratory experimental data collected in the “Cold Plate” apparatus, a new substrate resistance to soil evaporation is introduced. Moreover, previous functions to calculate plant resistance for transpiration calculation are evaluated and the functions that best approximate the measured values are selected. These two steps are important for correct evapotranspiration calculations and have not been done previously. Finally, the new green roof model is validated using quasi-steady state experimental data from the “Cold Plate.” The validation shows that the model tends to predict most of the heat and mass transfer appropriately, but tends to underestimate maximal evapotranspiration. Further research on convective heat transfer on plants is recommended, as well as a spectral reflectivity measurement of the substrate to improve the accuracy of the model. The final step before model implementation into a building energy simulation will be a dynamic validation using detailed laboratory and field data.