Hashem Akbari

Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas

Abstract Elevated summertime temperatures in urban ‘heat islands’ increase cooling-energy use and accelerate the formation of urban smog. Except in the city’s core areas, summer heat islands are created mainly by the lack of vegetation and by the high solar radiation absorptance by urban surfaces. Analysis of temperature trends for the last 100 years in several large U.S. cities indicate that, since ∼1940, temperatures in urban areas have increased by about 0.5–3.0°C. Typically, electricity demand in cities increases by 2–4% for each 1°C increase in temperature. Hence, we estimate that 5–10% of the current urban electricity demand is spent to cool buildings just to compensate for the increased 0.5–3.0°C in urban temperatures. Downtown Los Angeles (L.A.), for example, is now 2.5°C warmer than in 1920, leading to an increase in electricity demand of 1500 MW. In L.A., smoggy episodes are absent below about 21°C, but smog becomes unacceptable by 32°C. Because of the heat-island effects, a rise in temperature can have significant impacts. Urban trees and high-albedo surfaces can offset or reverse the heat-island effect. Mitigation of urban heat islands can potentially reduce national energy use in air conditioning by 20% and save over

Peak power and cooling energy savings of high-albedo roofs

10B per year in energy use and improvement in urban air quality. The albedo of a city may be increased at minimal cost if high-albedo surfaces are chosen to replace darker materials during routine maintenance of roofs and roads. Incentive programs, product labeling, and standards could promote the use of high-albedo materials for buildings and roads. Similar incentive-based programs need to be developed for urban trees.

Long-term Performance of High-Albedo Roof Coatings

Abstract In the summers of 1991 and 1992, we monitored peak power and cooling energy savings from high-albedo coatings at one house and two school bungalows in Sacramento, California. We collected data on air-conditioning electricity use, indoor and outdoor temperatures and humidities, roof and ceiling surface temperatures, inside and outside wall temperatures, insolation, and wind speed and direction. Applying a high-albedo coating to one house resulted in seasonal savings of 2.2 kWh/d (80% of base case use), and peak demand reductions of 0.6 kW. In the school bungalows, cooling energy was reduced 3.1 kWh/d (35% of base case use), and peak demand by 0.6 kW. The buildings were modeled with the DOE-2.1E program. The simulation results underestimate the cooling energy savings and peak power reductions by as much as twofold.

Practical issues for using solar-reflective materials to mitigate urban heat islands

Abstract ooling energy savings of 10 to 70% have been achieved by applying high-albedo coatings to residential buildings in California and Florida. Since dirt accumulation can alter the performance of high-albedo roofs as an energy efficiency measure, we examined some high-albedo coatings at various stages of exposure to determine the magnitude of this effect. We conclude that most of the albedo degradation of coatings occurred within the first year of application, and even within the first two months of exposure. On one roof, 70% of the drop in albedo for the entire first year occurred within the first two months. After the first year, the degradation slowed, with data indicating small losses in albedo after the second year. We use measured data to estimate the effects of weathering of white roofs on seasonal cooling energy savings and estimate a 20% reduction from first year energy savings for all subsequent years (2–10). Although washing the roofs with soap is effective at restoring original albedo, calculations show that it is not cost-effective to hire someone to clean a high-albedo roof only to achieve energy savings. Instead, it would be useful to develop and identify dirt-resistant high-albedo coatings.

Cooling energy savings potential of reflective roofs for residential and commercial buildings in the United States

Solar-reflective or high-albedo, alternatives to traditionally absorptive urban surfaces such as rooftops and roadways can reduce cooling energy use and improve urban air quality at almost no cost. This paper presents information to support programs that mitigate urban heat islands with solar-reflective surfaces: estimates of the achievable increase in albedo for a variety of surfaces, issues related to the selection of materials and costs and benefits of using them. As an example, we present data for Sacramento, California. In Sacramento, we estimate that 20% of the 96 square mile area is dark roofing and 10% is dark pavement. Based on the change in albedo that is achievable for these surfaces, the overall albedo of Sacramento could be increased by 18%, a change that would produce significant energy savings and increase comfort within the city. Roofing market data indicate which roofing materials should be targeted for incentive programs. In 1995, asphalt shingle was used for over 65% of residential roofing area in the U.S. and 6% of commercial. Built-up roofing was used for about 5% of residential roofing and about 30% of commercial roofing. Single-ply membranes covered about 9% of the residential roofing area and over 30% of the commercial area. White, solar-reflective alternatives are presently available for these roofing materials but a low-first-cost, solar-reflective alternative to asphalt shingles is needed to capture the sloped-roof market. Since incoming solar radiation has a large non-visible component, solar-reflective materials can also be produced in a variety of colors.

Weathering of Roofing Materials-An Overview

We make quantitative estimates of the impact of roof reflectivity on cooling and heating energy use for buildings in the US. Prototypical buildings are simulated with reflective (light in color) and absorptive (dark in color) roofs. Differences of annual cooling and heating energy use and peak electricity demand between dark and light roofs yield the savings. The DOE-2 building energy simulation program is used for these calculations. Monetary savings are calculated using local utility rates. Savings are estimated for 11 US metropolitan statistical areas (MSAs) in a variety of climates. The total savings for all 11 MSAs are: annual electricity savings, 2.6 terawatt hours (TWh); net annual savings,

The long-term effect of increasing the albedo of urban areas

194 M; and peak electricity demand savings, 1.7 gigawatt (GW). Extrapolating the savings from the 11 MSAs to the entire United States, we estimate annual electricity savings of about 10 TWh and a net savings of about

Radiative forcing and temperature response to changes in urban albedos and associated CO 2 offsets

750 M in annual energy payments. Peak electricity power reduction is estimated to be about 7 GW.

Local climate change and urban heat island mitigation techniques – the state of the art

Deliverable for CEC Task 2.6.4. Report LBNL-59724. Submitted to Construction and Building Materials, February, 2006 Weathering of Roofing Materials-An Overview Paul Berdahl, Hashem Akbari, and Ronnen Levinson Lawrence Berkeley National Laboratory Berkeley, CA 94720 and William A. Miller Oak Ridge National Laboratory Oak Ridge, TN 37831 Abstract An overview of several aspects of the weathering of roofing materials is presented. Degradation of materials initiated by ultraviolet radiation is discussed for plastics used in roofing, as well as wood and asphalt. Elevated temperatures accelerate many deleterious chemical reactions and hasten diffusion of material components. Effects of moisture include decay of wood, acceleration of corrosion of metals, staining of clay, and freeze- thaw damage. Soiling of roofing materials causes objectionable stains and reduces the solar reflectance of reflective materials. (Soiling of non-reflective materials can also increase solar reflectance.) Soiling can be attributed to biological growth (e.g., cyanobacteria, fungi, algae), deposits of organic and mineral particles, and to the accumulation of flyash, hydrocarbons and soot from combustion. 1. Introduction Roofing materials are exposed to the elements, namely wind, sunlight, rain, hail, snow, atmospheric pollution, and temperature variations and consequently degrade over time. Even the most durable materials are modified by deposition of ambient dust and debris, and may provide an opportunity for colonization by biological organisms such as cyanobacteria, fungi and algae. In this paper, we broadly review how weathering occurs and discuss several engineering strategies that are employed for improving the performance of roofing materials. We have been engaged in a multi-year project to develop and commercialize cooler, solar-reflective roofing in conjunction with a number of industrial partners. These materials can save energy used for air conditioning and improve occupant comfort. Since reflective white materials are sometimes not suitable from an architectural standpoint, the work has included materials with specified visual colors but with high near-infrared reflectance [1]. This paper is a summary of what we have learned concerning the weathering of roofing materials.

A surface heat island study of Athens using high-resolution satellite imagery and measurements of the optical and thermal properties of commonly used building and paving materials

Solar reflective urban surfaces (white rooftops and light-colored pavements) can increase the albedo of an urban area by about 0.1. Increasing the albedo of urban and human settlement areas can in turn decrease atmospheric temperature and could potentially offset some of the anticipated temperature increase caused by global warming. We have simulated the long-term (decadal to centennial) effect of increasing urban surface albedos using a spatially explicit global climate model of intermediate complexity. We first carried out two sets of simulations in which we increased the albedo of all land areas between 20 and 45 latitude respectively. The results of these simulations indicate a long-term global cooling effect of 3 10 15 K for each 1 m 2 of a surface with an albedo increase of 0.01. This temperature reduction corresponds to an equivalent CO2 emission reduction of about 7 kg, based on recent estimates of the amount of global warming per unit CO2 emission. In a series of additional simulations, we increased the albedo of urban locations only, on the basis of two independent estimates of the spatial extent of urban areas. In these simulations, global cooling ranged from 0.01 to 0.07 K, which corresponds to a CO2 equivalent emission reduction of 25‐150 billion tonnes of CO2.