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Urban overheating and impact on the built environment

The present article aims to analyse and discuss the quantitative and qualitative impact of ambient and urban overheating in the built environment and in particular : on the peak electricity demand, the cooling and heating energy demand of buildings, the electricity consumption during the hot period, the energy impact on the electricity generation and supply systems and the transmission networks, the direct and indirect effects on outdoor air environmental quality as well as the energy and environmental impact on low income and vulnerable population. The existing literature on the topic, more than 250 published articles, has been analysed, classified and discussed. All results and conclusion are tabulated.


Introduction
Urban heat island is the most documented phenomenon of climate change. Increased ambient temperatures are experimentally documented in more than 400 major cities around the world. The average amplitude of urban overheating is between 4 to 5 C, while values up to 10 C are reported fr several cities. Parallel, to the local warming, the global climatic change increases the frequency and the amplitude of heat waves and the other extreme climatic phenomena. Several studies have shown that there is a synergetic effect between the local and global climate change resulting in a significant increase of the amplitude of the urban overheating. Future projections foresee a very significant global overheating, that may have a tremendous impact on the built environment and put under serious stress urban citizens and energy infrastructures, (1).
Urban and ambient overheating have a serious and well quantified impact on the global built environment. It is well documented that increase the cooling energy consumption of buildings, (2), raises the peak electricity demand during the hot period and oblige utilities to build additional power plants and increase the cost of electricity supply, (3), increase the concentration of harmful pollutants like the tropospheric ozone and the emission of pollutants by the power plants, (4), have a serious impact on the quality of life of vulnerable and low income population and threat their health, (5), and increases significantly the heat related mortality and morbidity, (6).
Numerous studies have been performed and published aiming to document and quantify the specific impact of the current and future urban and ambient overheating in the built environment. The present paper, has collected and analysed almost all of the known existing studies has assess their conclusions and results and has classified the reported quantitative impact in a comprehensive way.

Energy Impact of Urban and Ambient Overheating
The various implications of ambient and urban overheating in the built environment are summarised in Table 1 and ambient overheating on the energy demand of buildings as well as on the electricity generation and supply systems and the associated transmission networks, is given in Table 2. It is also estimated that the impact of the actual overheating increases in average the peak electricity demand during the warm period by an equivalent of 21.9 W per person and per degree of temperature increase. A very significant impact is also found regarding the systems of electricity generation and supply. Overheating reduces significantly the performance of the thermal power plants and limits considerably the capacity of the transmission systems, while it increases dramatically the cost of the necessary investments to avoid power supply problems. Table 2. Quantified Implications of ambient and urban overheating on demand and supply side components of electricity. Adapted from (7).

Demand Side Component
Overheating Effect Average Implications Increased ambient temperatures and extreme phenomena, may cause 14-23 % additional investments on electricity capacity in USA, relative to a non -climate change scenario for the years between 2010-2055, Assessment of the potential future impact of global and local warming on the demand and supply energy systems, shown that it is extremely high and may affect considerably the whole balance and economics of the energy sector

The Impact of Local and Global Overheating on the Environment and the Ambient Air
Quality Higher ambient temperatures act as catalysts and result in higher concentration of harmful pollutants like the tropospheric ozone. Table 3, reports the result of more than 50 published studies aiming to assess and quantify the specific direct and indirect impacts of ambient overheating on the global environmental quality. As it concerns the direct effect of overheating on air quality, this is mainly focussed on the increase of the ozone concentration at the ground level. Ozone is toxic atmospheric gas and may damage significantly the human health. As shown, during heat waves the concentration of ozone is found to increase significantly and up to 20 %, while the number of ozone episodes above the allowed threshold concentration is increasing considerably. A serious future increase of the concentration of the ground level ozone is also foreseen by many studies as a result of the expected global climatic change. As it concerns the indirect effects of the ambient overheating, several studies have demonstrated that the emission of harmful pollutants by the power plants is increasing significantly during the hot season. A considerable future increase of the emitted pollutants by the electricity generation systems is foreseen by several studies considering that the used fuels will not change significantly. Table 3. Quantified Implications of ambient and urban overheating on ambient air quality, Adapted from (7).

Air Quality Component
Overheating Effect Implication

Impact of Overheating on Low Income and Vulnerable Population
Increased ambient temperatures have a serious impact on the indoor environmental quality of low income households while may increase considerably the necessary cooling energy demand to satisfy the additional cooling needs induced by the ambient overheating. It is well known that low income population lives in less protected houses presenting a non-appropriate energy and environmental performance. Increase of the ambient temperature obliges low income population to consume additional energy to achieve indoor comfort or to live under unacceptable indoor conditions and very high indoor temperatures. Table 4, summarises the conclusions and results of almost 40 published studies aiming to assess experimentally the impact of overheating on the energy and environmental quality of low income and vulnerable population. As shown, the few low income households able to cover the cost of electricity for cooling purposes, has to spend almost the double budget than the average population because of the poor housing and energy conditions.  6 However, most of the studies shown that vulnerable population is not able to satisfy the cooling needs and is forced to live under very high indoor temperatures. As reported, indoor temperatures recorded in numerous monitoring campaigns, may reach or exceed 40 C, while extremely long spells with indoor temperature continuously above 30 C are recorded and reported. Exposure to high indoor temperatures put in risk and threatens the life of the vulnerable population while it increases dramatically the heat related mortality and morbidity especially in aged population groups.

Conclusions
Urban and ambient overheating has a very serious impact in the built environment. This is documented through numerous studies that have quantified the specific impact on the peak electricity demand, absolute cooling energy consumption, performance of supply and distribution networks, outdoor environmental quality and also the energy and social impact on low income and vulnerable population. In parallel, a very important research is carried out to identify and document the impact of high ambient temperatures on heat related mortality and morbidity.