Green Roofs - Living Architecture Monitor - Winter 2009 - (Page 20)

RESEARCH GREEN ROOFS AND LOCAL TEMPERATURE HOW GREEN ROOFS PARTITION WATER ENERGY AND COSTS IN URBAN ENERGY AIR CONDITIONING BUDGETS By Paul S. Mankiewicz, Ph.D., Peter Spartos & Eric Dalski W ater regulates temperature on the earth. Together with vegetation in the terrestrial landscape, vegetation can act to move heat out of the lower atmosphere in the form of vapor phase water. Energy and water costs are increasing worldwide, as is average global temperature. Since installation and maintenance costs of energy generation and distribution infrastructure are also increasing, together with the cost of fuel, green infrastructure may itself offer the means to increase efficiencies and decrease overall costs. Direct comparisons between standard and green infrastructure need to be made to inform specific project choices for buildings as well as regulatory frameworks. A major question: Is it more cost-effective to utilize potable water for cooling than electricity? This is a pressing issue, since much of energy consumption is utilized to regulate the temperature within buildings. While most space heating depends directly on combustion and distribution of warm fluids, most available cooling technologies depend on phase change and heat partitioning. Air conditioners work to make interior space cool by discharging heat to exterior spaces. This process itself increases the air conditioning costs of neighbors, since hvac efficiencies depend on input air temperature. Central to this comparison is the fact that virtually all electricity has a substantial water cost for its production. The water cost of electricity can be compared to green roofs that are supplied with water and that carry out just such thermal partitioning, described by the dimensionless Bowen Ratio, defined as the quantity or flow of sensible heat divided by latent heat. Between three and eight times as much energy is moved by plants through evaporation compared to convective heat loss. This means that two-thirds to more than three fourths LIVING ARCHITECTURE MONITOR WINTER of radiation striking vegetation with available water does not heat up urban landscapes that incorporate green roofs, but instead, local temperature is decreased below ambient temperature. ENERGY BALANCE To understand the impacts of vegetation and water on the thermal behavior of a green roof it is necessary to compare the components into which energy flows at the earth’s surface: RN H G LE R P The radiation load on an area (Rn), minus sensible heat (H), minus heat moved into air, soil and water (G), minus evaporative heat loss (LE), respiration (R) and photosynthesis (P) equals zero . In other words, energy inputs are partitioned between sensible and latent heat, respiration and photosynthesis. It is critical to understand, however, that the majority of this energy, approaching 99 percent, is dissipated as sensible and latent heat. The proportion of sensible to latent heat, the Bowen Ratio, is the critical value here, however, since it translates directly into temperature on the ground and in the lower atmosphere. For local air condition, this ratio thereby substantially impacts the quantity of electricity usage as well as the impact on local environmental quality. 20

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Winter 2009

Green Roofs - Living Architecture Monitor - Winter 2009
From the Founder: When is a Green Roof Really "Green"?
Strata - People, Products & Projects: Funding for the Green Infrastructure Foundation (GIF)
An Integrated Approach to Green Roofs
On the Roof with...
Project: A Green Wall for Wildlife
Research: Green Roofs and Local Temperature
Turning Your Roof into a Water-Saving "Living Oasis"
Feature: Water & Georgia
GRHC Update: GHRC Professional Development Calendar
Accreditation Exam Subject to Stringent Standards
Welcome New Corporate Members
On Spec: Change We Can Believe In

Green Roofs - Living Architecture Monitor - Winter 2009