Construction materials within the urban landscape contain impervious compounds and contribute to the heat island effect. Thermal conductivity and heat capacity are essential for heat storage, heat is absorbed via high thermal conductivity properties while high heat capacity stores heat within the fabric of a building. Combining both properties is referred to as thermal diffusivity and calculated by dividing a materials thermal conductivity with the heat capacity.
Rural areas contain lower thermal diffusivity while urban areas include high thermal diffusivity materials. Building materials utilised within the urban environment are constructed of dark elements such as concrete, tarmac and stone/marble building facades and such materials are considered low albedo absorbing high levels of solar radiation. Construction materials within the built environment impact on the thermal performance of buildings. Building fabrics act as heating batteries during the day and release heat at night thus contributing to increased temperatures within cities.
Heat capacity is the heat required to increase the temperature of a material by 1Cº and expressed in Wh/m3 ºC. The quantity of heat necessary to raise the temperature of a material by 1°C. During the day the surface of the building can exceed the adjacent air temperature. Increased temperatures affect commercial buildings within the urban environment and increase the requirement for mechanical cooling in summer months thus increasing energy demand.
Temperatures are being affected as a result of climate change, especially in summer months, the demand for cooling rises 2% for every 0.6ºC temperature rise, temperatures of 20-25Cº increases cooling demand by 5-10%. The UHI effect may double thermal loads in commercial buildings, thus affecting the coefficient of performance (COP) of HVAC systems, it is estimated that the COP may increase by 25%, thus reducing the efficiency of condenser units. Globally, Cities consume 60-80% of the worlds energy demand and are responsible for equally emitting the same percentage of Greenhouse gases; it is expected that the current trend will continue due to Urbanisation growth.
In 1950, 30% of the global population resided in cities and by 2030 sixty percent of the worldwide population will inhabit cities. Urbanisation is rapidly expanding and changing communities within the urban environment. Engineers and scientists are facing significant challenges in how to support growth in urban environments while maintaining a sustainable future. Presently urban areas are increasing to 67 million people per year equating to 1.3 million every week and most population growth will occur in arid regions. Urban areas within developing countries are expected to grow at an average rate of 2.4% annually equating to twice the rate in developed countries.