Thursday, January 22, 2009


Heat Island Mitigation Strategies: The Role of Urban Planning

Till today, urban developers and policy makers are not serious on the implications of the worsening heat environment. The costs as discussed above, are tremendous which would force this effect to be taken seriously into up-coming days. On one hand, there are certain things that might be difficult to change such as urban thermal mass, weather patterns and surface roughness. Elimination of these effects would require complete and drastic new way of thinking in the way cities are built and operate. But on the other hand, there are plenty of corrective measures that can be taken within the existing urban set-up such as increasing vegetation cover, albedo modification, efficient energy consumption and management of heat discharge sources which are possible by supportive urban planning and policy measures.

Increasing vegetative cover

Tree plantation is the most obvious and the easiest way to improve heat environment in existing urban set-ups. Trees help in a number of ways; they provide direct shade to the buildings from solar radiation so that less radiation will reach to the building walls, windows and roof to be absorbed. They also create shades in the soil and concrete pavements to act as heat sink for the buildings and asphalt roads. Increase in water vapor due to evapotranspiration by plant leaves is significant in taking the heat away. Trees also act as pollutants, carbon and noise sink. It helps to mitigate greenhouse effects by consuming carbon dioxide in the photosynthesis process. It is estimated that a street lined with trees can reduce dust particles of about 7,000 particles per liter of air. However, care must be used in choosing the type of trees since some trees give off organic compounds (hydrocarbons) into the atmosphere and contribute to ozone in forming smog.

Planting programs can help reduce urban temperatures and make cities greener. Within ten to fifteen years – the time it takes a tree to grow to a useful size – trees placed in strategic locations can reduce heating and cooling costs by an average of 10-20%. Over their lifetimes, trees can be much less expensive than air conditioners and the energy needed to run them.

Well-distributed green parks and water bodies around the urban city act as recreational and aesthetic beauty. Urban planners are concerned with parks and water bodies but their motivation is for aesthetic beauty rather than betterment of heat environment. In the existing urban set-up, metropolitan authorities could encourage green belts around the roadside and plantations. This strategy depend on the local climate condition whether the place of concern is hot-dry or hot-humid in nature. In the hot-dry regions, the evaporation from the soil is minimal, urban parks and water bodies increase water evaporation from both the plants and the soil, consequently the effect on local climate could be significant and desirable. On the contrary, hot-humid regions have low specific evaporation and reduction in the wind speed near the ground is undesirable from the comfort viewpoint (Givoni, 1997).

Albedo modification

Albedo is defined as the ability of the surface to reflect solar radiation. It is different from reflectivity in the sense that reflectivity might only account for visual bands whereas albedo accounts for all the incoming radiation to the surface. It is basically hemispherical reflection of radiation integrated over the solar spectrum (0.3 – 2.5 mm) and includes specular and diffuse reflection (Bretz et al, 1998). Asphalt roads, concrete pavements and corrugated roofs have low values of albedo which form the major part of the dense mega-cities. Low albedo surfaces absorb significant proportion of the solar radiation and contribute in worsening urban heat environment. The mitigation strategy therefore is to improve over-all albedo of the urban surfaces.

Improving the urban albedo, such as for buildings and other surfaces have additional advantages. Apart from facilitating urban surfaces to reflect most of the solar radiation, it also contributes in cooling the buildings so that air-conditioning demand is greatly reduced. Studies have shown that the cooling energy savings from the high-albedo roofs and walls in the buildings are very significant. Any heat island mitigation strategy would be required to identify the opportunities that exist in improving the urban surface albedo. The surface albedo property can be greatly enhanced either by mixing it with some third material that can greatly increase its albedo or replacing the traditional construction material completely. The “cool construction materials” can be used to improve solar reflectance without significant cost additions. The choice of light and white colored surfaces is possible, however, a distinction between the light colored surface and high albedo surface should be well understood since light colored surface only means high reflectivity in the visible band.

The effect of albedo modification by one or combination of various methods at the scale of a city and their implication to the overall temperature is not very much studied. In general, the motivation for such albedo improvement has been observed from the air-conditioning viewpoint at building scale rather than reduction of overall thermal situation at the city scale. Building owners, builders and architects have choice to select color of the rooftops, type of construction materials and other measures. Urban planners and policy makers can change the attitude of the stakeholders by improving building codes with thermal considerations, energy management and appropriate urban planning.

Efficient energy consumption and management of heat discharge sources

Since mega-cities are characterized by high energy consumption, ample opportunities exist to manage energy and the heat discharge sources. As stated earlier, air-conditioning is the major stationary heat discharge sources arising from buildings. Air-conditioning units discharge heat to the urban atmosphere continuously due to the energy consumption inside the buildings in various forms (mainly gas and electricity) and absorbed solar radiation through the building surfaces. Three types of management is important here. First, is to enhance energy efficiencies of the end use appliances and the way of supplying energy. Second, is the energy efficient building design from architecture standpoint. And third, is the location of heat discharge sources. High-rise buildings allow the flexibility of placing the air-conditioning units (or plants) at the height significantly above the ground surfaces and the prevailing wind at the height can effectively swipe away the heat without letting it to concentrate in the urban canopy. Although there could be concern on the costs that would conflict with the optimization of piping, a balance optimum is possible. A mixture of high-rise and medium rise buildings in the dense urban area also enhance the over-all urban ventilation by creating turbulence in wind canopy, the ventilation in such case might be better than the urban area with low density but with buildings of similar heights.

The effect of improving appliance efficiencies in buildings on urban heat environment might be very small without changing the way the energies are supplied into the buildings. A central air-conditioning system is energy and cost-wise more efficient than the smaller units in each rooms or at each floors in the multi-storey structures. District cooling is favorable in the dense urban structure. In individual detached homes, small measures such as shading of air-conditioning units can produce effective results.

Transportation is the major heat discharge source that is mobile and difficult to simulate. It is encouraging that the automobile fuel efficiency is improving but at the same time, concentration of vehicles and traffic congestion is also increasing in the mega-cities and the net effect of which is unfavorable from urban warming standpoint. An exact extent of automobile’s implication on urban heat environment is largely unknown. However, traffic management and reduction in the vehicle idle time in core city areas is expected to greatly relieve the heat island phenomenon.

The anthropogenic heat discharges in the big cities are significant. Major cities in the US are reported to have summer anthropogenic heating in the range of 20-40 W/m2 in comparison to the solar radiation of 700-1000 W/m2 for clear or partly cloudy day at noon (Taha, 1997).

In Los Angeles, the increased power costs the ratepayers about $100,000 per hour, about $100 million per year. It is estimated that about 1-1.5 gigawatts of power are used to compensate the impact of the heat island. Reducing the energy cost would also help in reducing the air pollution problem. By 2015, when the full implementation of reflective surfaces and vegetation comes in full-scale, the state will save about $4 billion per year in reduced cooling energy demand.

In order to combat urban heat island, the air quality has to be improved reducing the level of toxic gases, more trees to be planted, save energy and thus reduce pollution, and thereby save cost of energy and money, and improve the overall livability. Air quality management systems should include abatement and other measures to improve air quality, and to maintain air quality within a defined range. Enacting urban planning legislation to increase the amount of vegetation could see a reduction in temperatures. Another method is to reduce the amount of heat absorbed by civil structures by using construction materials that have high albedo and not prone to heat absorption.

The urban metabolism concept (Wolman, 1965) indicates that environmental quality improvement in urban areas rests on the careful use and removal of energy and matter. In the urban design sense, environment conscious urban designers can use at least three tools for the realization of the goals of energy efficiency, transport reduction and air quality improvement. These are thru zoning laws, building laws, and landscape control. Some attempts at utilizing these tools for the purposes of energy and transportation reduction have already been made (cf. Emmanuel, 1995). Although these attempts are from the temperate climate cities, they offer possible models for hot-humid cities.

In the enhancement of the urban physical environment, quality should be the major goal of climate-conscious design. In order to achieve the design goals of energy efficiency, transportation reduction and air quality improvement, in the tropics, design strategies could take one of the following forms:

Building form guidelines

Activity pattern controls

Control of relationship to natural features

Building Form

Court-yard forms


Activity Relationships for Comfortable Moving & Transport Reduction

Shopping Streets

Gathering Places

Provisions for Evening Life (Evenings are tropics' winter).

Pedestrian Paths and Nodes

Network for Cars

Relationship to Natural Features - Landscape Controls

Relationship to Waterbodies

Collection of Rainwater

Topographical Relationships

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