Impact on Energy Consumption

CHAPTER 3: RESILIENCE STRATEGIES: ROOF COOLING AND PASSIVE VENTILATION

3.1 Introduction

UHI mitigation strategies vary from providing natural vegetation to cool roofs and cool pavements. Different strategies are suggested by different experts in the ield of urban planning, architecture, natural resource management and transportation. These strategies have an impact on both local and global climates. In addition to environmental beneits, these strategies also help in reducing the energy consumption for cooling and increase the thermal comfort in buildings and slums during summers. This chapter discusses different such strategies both for indoor and outdoor environment of a building, like cool roof and cool pavements, passive ventilation, insulation, etc. Local climate should be considered before deciding the suitable mitigation strategy or combination of complementary strategies for dealing with UHI.

3.2 Trees and Vegetation

Trees and vegetation are simplest way to reduce UHI effects. Trees help in increasing the albedo of the surfaces. Planting of trees and vegetation has both direct reduce CO 2 from atmosphere and indirect reduce energy consumption contribution in reducing CO 2 from the atmosphere [32]. Other study revealed that amount of CO 2 reduced due to indirect effect is considerably greater than the amount consumed directly in the photosynthesis of plants [33]. Trees and vegetation improve the air quality and also lower surface and air temperatures by providing shade and through evapotranspiration. They are useful as a mitigation strategy when planted in strategic locations and use of appropriate species of plant around buildings and on the streets. .

3.3 Green Roofs

A green roof is a vegetative layer grown on a rooftop. Green roof reduces the surface temperature and provides shading, like trees and vegetation and reducing the air temperature by Evapotranspiration mechanism. Green roof temperatures depend on the roof’s composition, moisture content of the growing medium, geographic location, solar exposure, and other site-speciic factors [34]. Green roofs can be installed on various buildings like industrial, public and private. There are two kinds of green roof based on plant density and vegetation: 1. Extensive and 2. Intensive. In Germany, Green space is used as a premium in urban growth to limit the urban sprawl and developers are often required to provide green roofs as a compensation of lost open space [35]. Green roofs ilter particulate matter from the air and a study estimated that 93 m2 green roof can remove about 40 pounds 18.15 kgs. of PM from the air in a year, while also producing oxygen and removing carbon dioxide CO 2 from the atmosphere [36].

3.4 Cool Roofs and Cool Pavements

In most urban areas, roofs and pavements constitute almost 60 of the land surface area [37]. Most of the solar energy gets absorbed by these paved surfaces and roofs, increasing the roof and ambient air temperatures. A dark colour roof absorbs more solar energy in comparison to the light colour roof. The higher the relectivity and emissivity of the roof material, the less likely it is to store the heat and radiate it back into the building through the walls and roof [38] [39]. The observed maximum 50 O K temperature difference between surface and ambient air for low albedo surfaces while for high albedo surfaces, this temperature difference was found to be only 1 O K [40]. High absorptive and less emissive surfaces contribute in keeping the ambient air temperature high and thus indirectly increasing the cooling demands of the buildings. It was estimated that retroitting with cool roof options, 100 m 2 of roof offsets 10 tons of CO 2 emission. They also estimated that permanently retroitting urban roofs and pavements in the tropical and temperate regions of the world with solar-relective materials would offset 44 billion tons of emitted CO 2 , worth 1.1 trillion at 25ton [37].

3.5 Passive Ventilation

Passive ventilation is the most cost effective means for achieving thermal comfort. Before airtight houses and mechanical ventilation systems, people used to keep their houses cool by regulating the air low inside the building with the help of appropriately positioned window openings and thermal mass. These methods were traditionally part of the building design. Passive cooling systems use the minimum or no mechanical energy to keep the buildings cool. Passive ventilation can be deined in two ways 1.Pressure system and 2.Buoyancy system. Pressure systems rely on positive pressure on the windward side of the building causing a lower pressure on the leeward side, in turn creating air movement through the building. Hot air becomes light and rises up. The buoyancy system works on principle of removing this light hot air through ventilators in buildings. SCHEMATIC DIAGRAM OF URBAN HEAT ISLAND MITIGATION STRATEGIES IN BUILDINGS Planting of Vegetation Shading Devices Building Envelope Energy Eficiency Alternatives to air conditioning Building Protect from solar radiation Minimize heat iniltration Reduce anthropogenic heat Maintain comfortable thermal environment Movable Optimum insulation Energy Eficient lighting Ground coupled heat exchanger Natural and controlled mechanical ventilation Radiant cooling system Solar cooling Energy eficient household appliances Energy eficient ofice equipments Air tightness Relective materials Thermal inertia Fixed Green Roofs Green Walls Vegetation around buildings Adopted from Melissa Giguere [32] F5 Solar Relectance Solar Emittance Solar Relective Index Thermal Comfort Life Expectancy Types of Slope Elastomeric Acrylic Coating water based Elastomeric Acrylic Coating solvent based Cementitious Coating Spray Polyurethane Foam Heat Insulation Tiles Clay and Concrete Tiles Thermoset Membrane Thermoplastic Membrane Modiied Bitumen Metal Cool Rooing Built-up Rooing Extensive Green Roof Intensive Green Roof Roof Pond Cooling System Roof Mist Cooling System Thatched Roof 0.7 - 0.85 0.7 - 0.85 0.7 - 0.85 0.7 - 0.85 0.77 0.7 0.7 - 0.8 0.7 - 0.8 white 0.4 - 0.6 colour 0.6 - 0.75 0.5 - 0.7 0.5 - 0.8 Low Low Low High Low 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 0.94 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 0.8 - 0.9 Low Low Low Medium Low 100 - 112 100 - 112 100 - 112 100 - 112 96 92 - 100 92 - 100 92 - 100 69 - 93 55 - 85 55 - 100 Low Low Low High Low 3 - 8 o C 3 - 8 o C 3 - 6 o C 3 - 6 o C Good Good Good Good Medium Low to High Medium to High High High 2 - 4 o C 2 - 5 o C Medium to High 6- 7 years 7 - 10 years 7 - 10 years 5 - 7 years 50 years 50 years 10 - 20 years 20 - 30 years 10 - 30 years 20 - 50 years 10 - 30 years 5 years 10 - 15 years 5 years 5 years 5 years Low Steep Low Steep Low Steep Low Steep Steep Steep Low Steep Low Low Low Steep - Up to 40 Up to 4 Low Low ROOF COOLING OPTIONS T1 Steep Earth Air Tubes Wind Tower Solar Chimney Passive Downdraft Evaporative Cooling Orientation of the Building Attic Ventilation: Whole House Fan False Ceiling Earth Berming Rat Trap Bond Masonry Building Insulation Rammed Earth Walls Exterior Shading Devices Shading: Landscaping ADVANTAGES LIMITATIONS Most effective in dry and hot climate Not effective during early morning and late evening Provide comfort by cooling the building structure at night Not effective during summer day time Beneicial in hot and dry weather Not effective in very humid region Most economical solution Site constraints in high density urban areas Most effective during early morning and late evening Noisy if not properly installed Reduces the heat load on HVAC system, thereby reduces the energy costs Very effective natural heat sink Water seepage problems can occur Cost effective alternative to conventional brickwork for better thermal and sound insulation Building services like electrical, plumbing and drainage have to be planned in advance Also improves weather prooing and sound resistance. Makes it dificult to ix heavy items on the insulated walls Very effective thermal mass - cool in summer and warm in winter Not feasible for high rise buildings Least cost solution for cutting heat gain into the building Applicable in all weather conditions Dense vegetation can make the air warmer near the building Ensures better indoor air quality and fresh air circulation for effective year round comfort Needs access to additional land for running underground tubes PASSIVE COOLING TECHNIQUES: ADVANTAGES AND LIMITATIONS T2