Cool Roofs at Pomona. Jeremiah Steuterman

Transcription

1 Cool Roofs at Pomona Jeremiah Steuterman

2 Intro How does the reflectivity of Pomona s roofs affect air conditioning use and can this be improved with the application of reflective coatings? A cool roof limits heat transfer through the roof. Can reduce air conditioning costs and save energy. Can reduce urban temperatures and combat problems associated with Urban Heat Islands. Pomona s roofs are already fairly efficient due to the thermal properties of clay tile, but energy savings could improve with the application of reflective coatings.

3 What is a Cool Roof? Typically white coating designed to improve reflection of near-infrared wavelengths. A cool roof has high solar reflectance and high thermal emittance. Albedo is the ratio of reflected radiation to incident radiation. Measured on a scale of 0-1 (black-white). Thermal emittance is the radiant emittance of heat. It is related to thermal emissivity (also measured on a scale from 0-1), which is the ability of a material to radiate heat at a particular wavelength.

4 What Cool Roofs Do Reduce heat transfer across the building envelope. Costs can be regained as energy savings in first few years. Lowers air conditioning costs, but can also raise heating costs. Most effective in warm sunny climates. Can reduce cooling costs by 10-20% (Akbari 1997).

5 Urban Heat Islands Urban areas are significantly hotter than rural areas. Most pronounced in the evening when temperature difference can reach 22 F. In urban areas Increase energy consumption. Increase severity of smog Increase heat related illnesses In rural areas Lengthen growing season. Increase monthly rainfall. An increase in urban roof albedo of 0.25 and increase in pavement albedo of 0.15 could offset of 57 Gt of CO 2. This is 1.9 years of global CO 2 emissions (Menon 2010).

6 Other Heat Island Strategies Green roofs, reflective pavements, and shade trees to name a few. Green roofs can reduce heat transfer and surface temperature as well or better than reflective roofs, but may require irrigation in dry climates. A tree planting program in LA could reduce cooling energy use by up to 25% (Akbari 2002).

7 Different Roof Types Built-up (flat), asphalt shingle, concrete, and clay tile. Graph shows temperature effect of Roof material type Insulation Radiant barrier

8 Cool Tiles High albedo pigment coatings can be applied to tile to raise albedo by as much as $/m 2 cost premium for reflective coating. In Southern California cost can be regained in energy savings in 5-7 years (Levinson 2007).

9 What does this mean in LA Located in a warm arid coastal plain. Moderate cooling needs and minimal heating. Prone to serious smog events. 1 K decrease in air temperature results in 5% decrease in the probability that ozone exceeds federal standards (Rosenfeld 1998).

10 At Pomona Most roofs at Pomona are barrel mission style clay tile. Design of old missions was intended to regulate temperature in hot arid climates. Clay has naturally high reflectivity and emissivity.

11 At Pomona Standard Flat Roof above Frary Albedo 0.5 Cool Roof on Art Museum Albedo 0.9

12 Conclusions Potential energy benefits are not large enough to warrant immediate retrofit. Dirt accumulation can lower energy savings. Energy benefits could be gained by installing reflective coatings during regular maintenance. Pomona not much of a heat island due to trees and green space.

13 Acknowledgements and References Acknowledgements Thanks to: Orlando Gonzalez, Char Miller, Richard Elderkin, Bowen Close, Marcus Bianchi, and Hashem Akbari. References Hashem Akbari, Sarah Bretz, Dan M. Km-n, James Hanford Peak power and cooling energy savings of high-albedo roofs Energy and Buildings 25 ( I997 ) Surabi Menon, Hashem Akbari, Sarith Mahanama, Igor Sednev, and Ronnen Levinson. Radiative Forcing and Temperature Response to Changes in Urban Albedos and Associated CO2 Offsets Environmental Research Letters 21 January Ronnen Levinson, Hashem Akbari, Joseph C. Reilly. Cooler Tile Roofed Buildings with Near Infared Non-White Coatings Building and Environment 42 (2007) Arthur H. Rosenfeld, Hashem Akbari, Joseph J. Romm TV, Melvin Pomerantz. Cool communities: strategies for heat island mitigation and smog reduction Energy and Buildings 28 (1998)