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Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof

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Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof

Hosseini, Mirata (2014) Cool Roofs Savings and Penalties in Cold Climates: the Effect of Snow Accumulation on roof. Masters thesis, Concordia University.

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Abstract

Utilizing a cool roof is an efficient way to reduce a building’s usage of cooling energy, although it may increase the usage of heating energy in winter. In cold climates, during the winter, the sun angle is low, days are short, the sky is often cloudy, and most heating occurs during early morning or evening hours when the solar intensity is low. In addition, the roof may be covered with snow for most of the heating season. All these factors lead to wintertime heating penalties for cool roofs that are lower than what is commonly thought.
We used DOE-2.1E to simulate energy consumption in several prototype office and retail buildings in four cold-climate cities in North America: Anchorage (AK), Milwaukee (WI), Montreal (QC), and Toronto (ON). The effects of sun angle, clouds, daytime duration, and heating schedules can be modeled with the existing capabilities of DOE-2. Snow on the roof provides an additional layer of insulation and increases the solar reflectance of the roof. To simulate four different types of snow on the roof, we defined a function consisting of the U-value and absorptivity of the roof on a daily basis. We used an average based on six years of meteorological data from the National Oceanic and Atmospheric Administration (NOAA) and Environment Canada to estimate the snow thickness on the roof.
In the very cold climate of Anchorage, AK, the simulated annual heating energy consumptions of the prototype old retail building with a dark (warm) versus a cool roof (without considering the snow) are 123548 and 125848 MJ/100 m2, respectively (showing a 2300 MJ/100 m2 penalty for the cool roof). These numbers reduce slightly to 123216 and 124409 MJ/100 m2, respectively (showing 1193 MJ/100 m2 penalty for the cool roof), when “late-winter packed” snow is considered. In this way, for an old retail building in Montreal, a cool roof can save up to $62/100 m2. For a new, medium-sized office building with electric cooling and natural gas as heating fuel, a cool roof would save $4/100 m2 in Montreal, $14/100 m2 in Milwaukee and Anchorage, and $10/100 m2 in Toronto. Cool roof also saves maximum of 37$/100 m2 for a retail store building in Toronto.
Cool roofs for the simulated buildings resulted in annual energy expenditure savings in all cold climate regions.
A cool roof also reduces the electricity peak demand of the building during the cooling season; this effect is considered to be a practical method to improve the reliability of grids and plants or to prevent unwanted electricity shutdown on hot summer days. Cool roofs can reduce the peak electric demand of the retail buildings up to 1.9 and 5.4 W/m2 in Toronto and Montreal, respectively.
Most HVAC systems are designed based on the peak summer cooling load. A cool roof can reduce the summer cooling load, which would lead to downsizing of HVAC systems. A downsized HVAC system can operate more efficiently throughout the year, including during the heating season.

Divisions:Concordia University > Gina Cody School of Engineering and Computer Science > Building, Civil and Environmental Engineering
Item Type:Thesis (Masters)
Authors:Hosseini, Mirata
Institution:Concordia University
Degree Name:M.A. Sc.
Program:Building Engineering
Date:22 September 2014
Thesis Supervisor(s):Akbari, Hashem
Keywords:Cool roofs, Cold climates, Energy savings
ID Code:979065
Deposited By: MIRATA HOSSEINI
Deposited On:03 Nov 2014 14:07
Last Modified:18 Jan 2018 17:48

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