Cool pavement

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Cool pavement is defined by the United States Environmental Protection Agency [1] as pavement that implements technologies contributing to heat island effect [2] reduction efforts. Most cool pavements either increase pavement albedo to reflect shortwave radiation out of the atmosphere and reduce heat transfer to the Earth’s surface, or use evaporative water cooling through the pavement to lower ambient temperatures. [3] Newer technologies involve energy harvesting, heat storage methods, and phase change materials. [4] Cool pavements are commonly made with reflective coatings or aggregates applied to conventional pavements or incorporation of porous or permeable materials. [4] While pavement load capability may be reduced with more frequent maintenance requirements, [4] cool pavements show promising results in reducing ambient temperatures and reducing energy usage. [5] [6]

Contents

Development

The United States Environmental Protection Agency defines cool pavement as pavement that implements technologies contributing to heat island effect reduction efforts. [1] By reducing the heat island effect, cool pavement limits the energy required to keep buildings cool, which can reduce greenhouse gas emissions. [2] Researchers at the Lawrence Berkeley National Laboratory predict that increasing solar reflectance of pavements to 35% from the standard 10% could lead to a 1 °F reduction in ambient temperature. [7]

Cities have observed reduced temperatures after implementing cool pavements and other green infrastructure. Rome has seen up to a 50% decrease in mean radiant temperature with shaded, high-reflective pavements. [8] Los Angeles, Phoenix, and Tokyo have also experimented with cool pavements, with Los Angeles laying out 181 lane-miles of solar-reflective coating. [9]

Research into cool pavement technologies is still in its early stages, with current work focusing on maintenance, effectiveness quantification, and cost-efficient large-scale implementation. [10]

Types

Evaporative pavements

Evaporative pavements are a widely used type of cool pavement that reduces surface and air temperatures through water evaporation, which cools both the surface as well as the surrounding air. [1] These pavements require moisture from rain or irrigation systems to function effectively and are engineered with permeable, water-absorbing materials such as soil additives and porous asphalt. [1] The continuous absorption and evaporation of water can weaken their structure over time, eventually leading to degradation of the pavement surface. [1] [11] However, evaporative pavements come with limitations including needing significant amounts of water which may be scarce in arid regions, and being generally less durable than traditional asphalt. [1] [11]

Reflective pavements

The effectiveness of reflective cool pavements is dependent on their ability to reflect solar radiation or albedo. [5] These pavements utilize reflective aggregates, advanced binders, and specialized coatings to increase solar reflectivity for reduced heat absorption, leading to lower surface and ambient temperatures. [1] Common coatings used are clear resin, light-colored aggregates, and light-colored cement, all of which vary in their reflective abilities. [12] Because they reduce temperature by reflecting sunlight, reflective pavements work to reduce surface temperatures only during the day. [1] Cement plays an important role in enhancing concrete’s reflective properties, with lighter shades improving solar reflection. [1]

Other types

Other types of pavements, known as ‘heat storage modified pavements,’ include energy-harvesting, high-conductive, and phase change material (PCM)-incorporated pavements. [4] [11] Energy-harvesting pavements harvest heat energy through the circulation of liquids, which can then be converted to electricity via thermoelectric generators. [4] Additionally, photovoltaic cells can be used to convert sunlight into electrical energy. [4] High-conductive pavements incorporate materials with high thermal conductivity to quickly transfer heat from the pavement surface to the soil below, which dissipates heat faster than the pavement material. [4] In contrast, PCM-incorporated pavements utilize materials that absorb, store, and release heat as they transition from solid to liquid states. [11] This process helps minimize extreme heat since it is a form of temperature regulation within pavement. [11]

Manufacturing

Evaporative pavements

Evaporative porous pavements can be manufactured by drilling vertical holes in standard interlocking concrete paver blocks and filling the holes with gravel. [4] The holes increase the pavement’s permeability and cooling properties by allowing water to accumulate. [4] This increase is also achieved by mixing in aggregates covered with cement paste and asphalt binder before pavement is laid, which creates connected pores that hold water. [4] Permeable and water-retaining pavements use a permeable material with pores grouted using blast furnace slag or pervious mortar to hold runoff water in a particular layer. [4]

Evaporative pavements tend to have lower mechanical strength than other pavements due to their voids. [4] Maintenance and replacement requirements are more frequent, as permeable pavements are more susceptible to water damage and raveling (pavement surface disintegration). [4] [5]

Reflective pavements

Reflective pavement manufacturing involves the application of a top coat onto finished pavement, or the mixing of reflective materials into wet concrete. [4] Coating materials include water or solvent based coatings with high solar reflectance properties, infra-red colored coatings, and thermochromic materials [13] which change color and optical properties based on ambient temperature. [4] Mixed reflective additives include thermochromic additions to asphalt binder, [13] heat reflective additives, and slag and fly ash in cement mixtures. [4]

Reflective pavements are prone to pollution which significantly decreases their effectiveness over time. [4] Maintenance on reflective pavements usually involves a surface coating or seal to compensate for surface wearing or damage over time. [5] Chip seals use pneumatic rollers to embed aggregates into pavement surface; sand and scrub seals inject additives into pavement cracks and roll them in; and microsurfacing involves spraying a high-friction, high-reflective coating over the road surface. [5]

Other types

Energy-harvesting cool pavements circulate fluid through the pavement to capture thermal energy via convection. [4] Fluid circulation is accomplished by embedding stainless steel, copper, or concrete pipes into the pavement and flowing air or water through the pipes. [4] Other energy-harvesting pavement manufacturing methods involve embedding photovoltaic cells, bismuth telluride-based thermoelectric generators, or pyroelectric materials into the pavement. [4] So far, energy-harvesting cool pavements have not been proven successful at handling heavy traffic, as the energy harvesting elements are prone to damage or decrease in efficiency after being subjected to road maintenance. [4] Manufacturing technologies to improve the load-bearing capabilities of energy harvesting pavements are still being developed. [4]

High-conductive cool pavements can be modified by adding materials with high thermal conductivity to asphalt. Materials include carbon or steel fiber, graphite, carbon black compound, steel slag, or reinforcing with metal rods. [4]

PCM-incorporated cool pavements are made by encapsulating the phase change materials before mixing them into asphalt, as direct contact with PCM materials can negatively affect the pavement’s mechanical strength and make it more fracture prone. [4] [11] PCM pavements are most commonly made via impregnation, where a porous material such as shale or clay is filled with and immersed in the PCM. [11] The porous material is then covered with cement before being mixed into the concrete or asphalt that makes up the pavement. [11] Encapsulation can also be accomplished by covering the PCM with a metal shell before mixing into asphalt. [4]

Safety implications

Cool pavement improves the road’s permeability, increasing the safety of drivers during wet seasons. [5] The porous composition of evaporative cool pavement allows water to penetrate through the road, which can increase tire traction and reduce water spray. [5] In addition, cool pavements can decrease tire noise by up to eight decibels and lower traffic noise levels to 75 dB. [11] [5]

Reflective cool pavements can also increase visibility at night, reducing the need for streetlights which makes streets safer and also limits energy consumption. [6] However, the enhanced albedo of the road can also reflect more light into drivers' eyes, creating a high glare that can impact visibility. [14] To combat this, anti-glare coatings can be incorporated into the road. [15]

Additionally, sunlight and high temperatures accelerate the production of ground-level ozone (smog), which is detrimental to humans and animals. [16] At least one out of three people in the United States experiences ozone health-related issues such as an irritated respiratory system, asthma, and a weakened immune system. [16] Cool pavements can combat this as increased albedo and reduced temperatures prevent nitrous oxides and volatile organic hydrocarbon gases from reacting and creating ozone. [17]

Societal implications

Installing cool pavements in urban heat islands can combat thermal inequity (unequal distribution of heat in urban areas) which disproportionately impacts minorities and low-income individuals. [18] These communities often lack the necessary resources to adapt to high temperatures. [18] During warm seasons, urban areas can reach temperatures that are 2-8 °F hotter, but cool pavements decrease surface and air temperatures by increasing the albedo of roads, improving comfort. [16] [10]

Environmental impact

Cool pavements can reduce local temperatures, leading to lower energy demands as air conditioners require less power to cool buildings. For instance, increasing pavement reflectivity (albedo) in Los Angeles has been estimated to save over $90 million annually in energy costs. [5]

Lower energy use translates to reduced greenhouse emissions and air pollution, depending on the fuel sources powering local grids. [19] Cooler temperatures can also slow down smog-producing chemical reactions, contributing to cleaner air. In 2007, researchers estimated that if global pavement albedo increased by 35 to 39 percent, it could lead to carbon dioxide reductions valued at approximately $400 billion. [20]

In 2022, a project installed over 700,000 square feet of reflective pavement in Pacoima, California, a city in Los Angeles County known for its high summer temperatures. [21] This project examined how cool pavements affect the local microclimate, assessing changes in surface and air temperatures, pedestrian comfort, and issues like glare and air quality. [21] On regular hot summer days, the average air temperature dropped by 0.2 °C to 1.2 °C, while surface temperatures were lower by 2.6 °C to 4.9 °C. [21] Results suggest that cool pavements can enhance comfort for pedestrians, with mean radiant temperature reductions between 0.9 °C and 1.3 °C, and physiologically equivalent temperature decreases from 0.2 °C to 1.7 °C. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Albedo</span> Ratio of how much light is reflected back from a body

Albedo is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 to 1. Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time.

<span class="mw-page-title-main">Urban heat island</span> Situation where cities are warmer than surrounding areas

Urban areas usually experience the urban heat island (UHI) effect, that is, they are significantly warmer than surrounding rural areas. The temperature difference is usually larger at night than during the day, and is most apparent when winds are weak, under block conditions, noticeably during the summer and winter. The main cause of the UHI effect is from the modification of land surfaces while waste heat generated by energy usage is a secondary contributor. Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population. As a population center grows, it tends to expand its area and increase its average temperature. The term heat island is also used; the term can be used to refer to any area that is relatively hotter than the surrounding, but generally refers to human-disturbed areas.

Energy-efficient landscaping is a type of landscaping designed for the purpose of conserving energy. There is a distinction between the embedded energy of materials and constructing the landscape, and the energy consumed by the maintenance and operations of a landscape.

In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation.

<span class="mw-page-title-main">Heat transfer</span> Transport of thermal energy in physical systems

Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation, and transfer of energy by phase changes. Engineers also consider the transfer of mass of differing chemical species, either cold or hot, to achieve heat transfer. While these mechanisms have distinct characteristics, they often occur simultaneously in the same system.

<span class="mw-page-title-main">Asphalt concrete</span> Composite material used for paving

Asphalt concrete is a composite material commonly used to surface roads, parking lots, airports, and the core of embankment dams. Asphalt mixtures have been used in pavement construction since the beginning of the twentieth century. It consists of mineral aggregate bound together with bitumen, laid in layers, and compacted.

<span class="mw-page-title-main">Permeable paving</span> Roads built with water-pervious materials

Permeable paving surfaces are made of either a porous material that enables stormwater to flow through it or nonporous blocks spaced so that water can flow between the gaps. Permeable paving can also include a variety of surfacing techniques for roads, parking lots, and pedestrian walkways. Permeable pavement surfaces may be composed of; pervious concrete, porous asphalt, paving stones, or interlocking pavers. Unlike traditional impervious paving materials such as concrete and asphalt, permeable paving systems allow stormwater to percolate and infiltrate through the pavement and into the aggregate layers and/or soil below. In addition to reducing surface runoff, permeable paving systems can trap suspended solids, thereby filtering pollutants from stormwater.

<span class="mw-page-title-main">Heat recovery ventilation</span> Method of reusing thermal energy in a building

Heat recovery ventilation (HRV), also known as mechanical ventilation heat recovery (MVHR) is a ventilation system that recovers energy by operating between two air sources at different temperatures. It is used to reduce the heating and cooling demands of buildings.

<span class="mw-page-title-main">Phase-change material</span> Substance with high latent heat of melting or solidifying

A phase-change material (PCM) is a substance which releases/absorbs sufficient energy at phase transition to provide useful heat or cooling. Generally the transition will be from one of the first two fundamental states of matter - solid and liquid - to the other. The phase transition may also be between non-classical states of matter, such as the conformity of crystals, where the material goes from conforming to one crystalline structure to conforming to another, which may be a higher or lower energy state.

The climate in urban areas differs from that in neighboring rural areas, as a result of urban development. Urbanization greatly changes the form of the landscape, and also produces changes in an area's air. The study of urban climate is urban climatology.

<span class="mw-page-title-main">Reflective surfaces (climate engineering)</span>

Reflective surfaces, or ground-based albedo modification (GBAM), is a solar radiation management method of enhancing Earth's albedo. The IPCC described this method as "whitening roofs, changes in land use management, change of albedo at a larger scale ."

<span class="mw-page-title-main">Waste heat</span> Heat that is produced by a machine that uses energy, as a byproduct of doing work

Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utility than the original energy source. Sources of waste heat include all manner of human activities, natural systems, and all organisms, for example, incandescent light bulbs get hot, a refrigerator warms the room air, a building gets hot during peak hours, an internal combustion engine generates high-temperature exhaust gases, and electronic components get warm when in operation.

<span class="mw-page-title-main">Sustainable drainage system</span> Designed to reduce the potential impact of development

Sustainable drainage systems are a collection of water management practices that aim to align modern drainage systems with natural water processes and are part of a larger green infrastructure strategy. SuDS efforts make urban drainage systems more compatible with components of the natural water cycle such as storm surge overflows, soil percolation, and bio-filtration. These efforts hope to mitigate the effect human development has had or may have on the natural water cycle, particularly surface runoff and water pollution trends.

<span class="mw-page-title-main">Passive cooling</span> Building design approach

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from entering the interior or by removing heat from the building.

<span class="mw-page-title-main">Thermal wheel</span> Type of energy recovery heat exchanger

A thermal wheel, also known as a rotary heat exchanger, or rotary air-to-air enthalpy wheel, energy recovery wheel, or heat recovery wheel, is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of air-handling units or rooftop units or in the exhaust gases of an industrial process, in order to recover the heat energy. Other variants include enthalpy wheels and desiccant wheels. A cooling-specific thermal wheel is sometimes referred to as a Kyoto wheel.

The environmental impact of concrete, its manufacture, and its applications, are complex, driven in part by direct impacts of construction and infrastructure, as well as by CO2 emissions; between 4-8% of total global CO2 emissions come from concrete. Many depend on circumstances. A major component is cement, which has its own environmental and social impacts and contributes largely to those of concrete.

Hashem Akbari is an Iranian-American professor of Architectural, Civil and Environmental engineering at Concordia University. He specializes in research on the effects of urban heat islands, cool roofs, asphalt paving materials, energy efficiency, and integrated energy optimization in building.

<span class="mw-page-title-main">Oasis effect</span> Alteration of a surrounding environment by the presence of a water source

The oasis effect refers to the creation of a local microclimate that is cooler than the surrounding dry area due to evaporation or evapotranspiration of a water source or plant life and higher albedo of plant life than bare ground. The oasis effect is so-named because it occurs in desert oases. Urban planners can design a city's layout to optimize the oasis effect to combat the urban heat island effect. Since it depends on evaporation, the oasis effect differs by season.

<span class="mw-page-title-main">Passive daytime radiative cooling</span> Management strategy for global warming

Passive daytime radiative cooling (PDRC) is the use of unpowered, reflective/thermally-emissive surfaces to lower the temperature of a building or other object.

Transpirational cooling is the cooling provided as plants transpire water. Excess heat generated from solar radiation is damaging to plant cells and thermal injury occurs during drought or when there is rapid transpiration which produces wilting. Green vegetation contributes to moderating climate by being cooler than adjacent bare earth or constructed areas. As plant leaves transpire they use energy to evaporate water aggregating up to a huge volume globally every day.

References

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