One of the goals to improve the sustainability of cities is to invest in developing the resilience (capability to handle and respond to various disturbances) in order to substantially reduce disaster damage to critical infrastructure and disruption of basic services, such as the supply of energy [1]. The energy sector can be evaluated through three issues: energy security, energy sustainability and energy equity (Energy Trilemma) [2]. In Italy, to reduce energy risks, prior actions remain energy efficiency and renewable energy sources issues encouraged also by public subsides [3]. The energy sector also shows emerging risks, these include technical disruptions, extreme weather events, and also citizen and consumer awareness and concerns about the localization and the use of energy infrastructure. In the last years, these risks had a growing impact on the energy sector through:

• Energy-water-food nexus (the interdependency of the human use of water, food and energy that impacts on economy, society, environment, health, and wealth);

• Extreme weather events (disastrous, unusual or not seasonal events);

• Cyber risks (offensive actions to maneuver infrastructure, information systems, computer networks and devices).

The resilience implies a functioning and stable system, providing continuity and minimizing interruptions to service. The concepts of resilience are tightly linked to sustainability and energy is one of the key features in urban sustainability. Then an energy resilient urban system should be able to ensure availability, accessibility, affordability and acceptability of energy supply, under varying conditions of uncertainty, through enhancing its ability to plan/prepare and absorb the effects, with a rapid recovery of the service with minimum costs [4]. In fact, the policies for resilient cities are those strengthening the urban system’s capacity to change in response to economic and natural shocks to the system. The various human-built systems of a city and the natural system are complex [5].

Availability denotes the existence of adequate supplies of energy resources and reserves and proper infrastructure for transforming them into energy services (i.e., diversification on energy sources).

Accessibility refers to the importance of spatial proximity of energy supply and demand.

Affordability refers to the capacity of population to meet their energy needs at reasonable costs (i.e., the stability/predictability of energy prices helps in ensuring energy affordability).

Acceptability refers to efficiency of energy systems (generation, transmission and distribution) minimizing environmental impacts (i.e., new clean technologies and smart systems).

Cities are the major energy consumers and the continuity of energy service is essential for their effective functioning. As energy is an important driver of current economic activities, it is important for countries to secure their energy sources. This is particularly true for EU countries in light of geopolitical considerations and ongoing reforms in the EU energy market [6]. This is primarily related to the rapid economic growth and at the same time, a high degree of dependence on import of energy generating products [7]. Access to energy is one of the most important aspects of the well-being and sustainable development of modern societies. The role of energy is directly linked to the economic, social and environmental development of a country [8]. Moreover, in urban areas, with the increase of population, the demand of energy will consequently increase. To ensure sustainability, precautionary measures should be taken in order to acknowledge future uncertainties and avoid potential risks caused by the low availability of energy sources.

Therefore, achieving sustainability requires an integrated approach that covers multiple, interrelated environmental, social and economic dimensions. Precautionary measures to respect uncertainty, promoting inter- and intra-generational equity, minimizing adverse impacts on the environment, maximizing efficiency and economic benefits through resource management and adopting bottom-up and participatory approaches are the main criteria used for assessing achievement of sustainability goals.

To address the energy risks, various energy-related aspects could be analysed, such as:

• Resources (energy, water-energy nexus, food-water-energy nexus);

• Land use, urban geometry and morphology (land use, urban morphology, urban design);

• Infrastructure (supply, transmission, distribution, backup and storage, green and blue infrastructures, transportation, innovation);

• Governance (management, monitoring and planning, standards and laws, subsides, fuel price);

• Socio-demographic aspects and human behavior (demographics, health and equity, behavioral aspects).

In the last years, mitigation and adaptation to climate changes should also be considered as further aggravating risks. If the climate change depends on energy-related emissions, the reduction of energy risks, with deep emission cuts across various sectors, will be also crucial for climate stabilization [9], [10].

In Figure 1, an energy resilient system has been represented by the four components (i.e. availability, accessibility, affordability, and acceptability) considering five themes for the interventions: urban infrastructures, resources, land use and urban morphology, urban governance, socio-demographic aspects and human behavior. In this study, the energy components have been evaluated considering these five themes, for the definition of an energy security index.

Figure 1.

An energy resilient system

The definition of energy sustainability is based on three core dimensions - energy security, energy equity and environmental sustainability. Taken together, they constitute a ‘trilemma’, and achieving high performance on all three entails complex interwoven links between public and private actors, governments and regulators, economic and social factors, national resources, environmental concerns and individual behaviors [11]. Therefore, the challenge for a future sustainable and smart energy sector will be represented by [12]:

• Energy security, the effective management of energy supply with reliable energy infrastructures to meet current and future energy demand;

• Energy equity, accessibility and affordability of energy supply across all population;

• Environmental sustainability, encompasses the achievement of supply and demand-side energy efficiencies and the development of energy supply from renewable and low-carbon sources.

The current European climate and energy policies EU-2020 and EU-2030/2050, demonstrate that climate changes, energy efficiency and renewable energy sources are currently influencing the energy sector due to a strong and constant political agenda on the road to a sustainable, inclusive, affordable and resilient energy system [12]. In this context, it must be considered that locally, specific low-carbon technologies or energy savings interventions appear preferential and then more suitable to reach EU objectives.

In this study, the concept of energy resilience is developed considering the following themes:

• Sustainable cities (environmentally, economically, and socially healthy and resilient cities);

• Energy efficiency (energy efficient systems and energy saving measures);

• Renewable energy (exploiting the available renewable energy sources with low greenhouse gas emissions);

• Smart grids (innovative models for demand-supply management).

Energy efficiency and renewable energy are also solutions to reduce energy dependency from abroad, ensuring energy security and reducing the energy risk, in line with the EU policies on energy and climate changes.

For the city of Turin, to promote a sustainable development, the trend of energy efficiency measures, the use of the available renewable resources and the reduction of CO2 emissions, together with the reduction of energy import dependency were analysed [12]. To address the issue of smart grids, a study on the expansion of the District Heating (DH) network has been provided, the development of integrated energy systems are seen as a key option to provide operational flexibility to the energy system and improve its overall efficiency to the thermal and electrical users [13]. The aim of this study is on energy systems and their impact on sustainability and to understand how to satisfy the high energy demand in complex urban contexts with low temperatures outdoor spaces through the available renewable energy sources [14], [15].

In the first phase of the work, an analysis of the characteristics of the buildings of the City of Turin and their energy consumptions from 2000 to 2013 is presented. Energy consumptions are analyzed taking into account renewable and non-renewable resources, types of users (residential and non-residential), CO2 emissions, microclimatic conditions (climate data from 6 weather stations) and socio-economic conditions (number of inhabitants, earning and income).

In the second phase of the work, the optimization of the existing energy supply network together with the potential of the available renewable solar technologies have been evaluated. The historical evolution of the DH network of the city of Turin was analyzed with the objective to develop future scenarios at 2030 and 2050. The results of this analysis will consent to evaluate current and future energy policies and the possible actions to achieve energy security and resilience reducing energy risks and vulnerabilities [16]. Energy security is important for all countries and substantially more important for countries that are simultaneously exposed to multiple supply vulnerabilities [8].

In the third phase of the work, the energy security index was calculated, considering different indicators and using the aggregation and weighting method [17]. Energy security is an extremely complex phenomenon that itself is neither clearly defined, nor are its limits clearly identified. Furthermore, the very energy security is affected by a large number of factors that are different in types and levels of influence, whereby most of them are very complex by their nature [7]. The energy security index consents to aggregate and to quantify more indicators as energy efficiency, environmental sustainability and energy equity to. To each of these indicators it is possible to give a certain weight according to its perceived importance and with the aggregation technique can be defined a composite index.

This paper shows a method to evaluate energy resilience in urban spaces, in particular how the energy system dependency can affect resilience capacities was investigated. The resilience of the system is its ability to reduce both the magnitude and duration of the deviation as efficiently as possible to its usual targeted system performance levels. As resilience increases, the degree of damage for a given intensity hazard decreases.

Three properties or capacities are used to define, quantify and design for better resilience [18]:

• Absorptive capacity, or the ability of the system to absorb the disruptive event;

• Adaptive capacity, or the ability to adapt to the event;

• Restorative capacity, or the ability of the system to recover.

The proposed method focuses on the concept of adaptive capacity, considering relationships between the different energy systems: the heating systems may have the adaptive capacities that allow the systems to reorganize themselves (using renewable energy sources or more efficient systems as DH) to reduce its dependency upon existing system (old inefficient boilers using gas oil or fuel oil). In this paragraph, the methodology applied to the case study of the City of Turin is presented.

Scenarios analysis

Starting from the study on the potential expansion of the DH network of Turin [20], [21], two scenarios were hypothesized. The first scenario considers the current trend of the development of the DH network and the second scenario considers the current development of the DH network plus the trend of installation of solar-thermal collectors.

Turin consumption data (MWh/year) and volumes served (m³/year) are provided by the DH Iren Company. These data were used to analyze the residential and tertiary volumes of heated buildings with a Geographic Information System (GIS) tool. The constraints for a future expansion of the DH network were also considered (Figure 2a): the presence of individual heating systems (28.4%), the territorial limits given by the Po river and the hills (12.9%), the high density historical old town in the center of Turin (8.1%), the non-connectable buildings with a volume smaller than 2,500 m³ (4.8%) and the diameter of the existing pipelines. In Figure 2a the limit on autonomous heating systems in yellow can be noted on the hill (South-East) and in the suburbs areas. Considering these constraints, the residential and tertiary volumes and number of buildings connectable and non-connectable to the DH network have been calculated (Figure 2b). The number of the connectable buildings to DH, in the medium-term scenario, will be 17,870, equal to 35,679,069 m³, in the long-term scenario, the number of the addition connectable buildings will be more than 6,558, equal to 12,802,847 m³.

Figure 2a.

DH network expansion constraints: individual heating systems (in yellow), Po river and hill (in green), historical centre (in red) and buildings with a volume < 2,500 m³

Figure 2b.

The expansion of the DH network starting from the existing network and the future expansion in the medium-term (marked with black outline) and long-term

For the medium-term to 2030, the volumes connected to the existing DH network, the potentially connectable volumes, the non-connectable volumes (due to territorial, technical and economic constraints) and total volumes have been considered (Table 1). Thermal consumptions and emissions (state of fact and future scenarios) have been calculated, including residential and tertiary volumes that could be connected to the existing DH. In 2030, only the buildings reachable from the existing network will be connected to the DH, in the long-term scenario at 2050 all the “connectable” buildings in Turin will be served by the DH network.

Table 1.

Volumes, consumptions and CO2 emissions analyses of connectable and non-connectable buildings by the DH network for the City of Turin

		Tot. res./tertiary buildings	Connectable to DH network	Non-connectable to DH network	Buildings connected to DH network
DH existing areas	Volume [m3]	87,226,609	35,679,069	10,733,500	40,814,040
	Consumption [MWh/y]	–	2,018,823	607,332	2,309,374
	Emissions [t CO2]	–	455,263	136,959	353,334
DH non-served areas	Volume [m3]	50,984,035	12,802,847	38,181,188	0
	Consumption [MWh/y]	–	724,421	2,160,400	0
	Emissions [t CO2]	–	194,419	474,710	0
Total	Volume [m3]	–	48,481,917	48,914,688	40,814,040
	Consumption [MWh/y]	–	2,743,244	2,767,731	2,309,374
	Emissions [t CO2]	–	649,682	611,669	353,334

Finally, the solar-thermal collectors trend of diffusion was also added considering a +20% of installations at 2030 and +30% at 2050. Currently in Italy, the market of solar collectors offers solar modules with efficiency of 0.8–0.85 and with a global system efficiency of 0.5-0.55 taking into account also the heat storage, exchange and distribution.

The City of Turin is located in an area of about 130 km², mostly flat. It is surrounded by the Alps and by the hills and it is crossed by four rivers (Po, Dora Riparia, Stura and Sangone). Turin’s climatic conditions are strongly influenced by the presence of the Alps, the climate is continental temperate, typical of mid-latitudes, with cold-dry winters and hot-wet summers. The Urban Heat Island (UHI) phenomenon influences the Turin climate with significantly higher temperatures in the city center and the lowest temperatures in the suburbs and in the surrounding rural and hilly areas. Considering the various weather stations in the city, there are differences in the outdoor air temperatures, mainly due to human activities (residential or industrial areas), the presence of UHI and the proximity of parks and rivers [25].

In this study, the residential and tertiary sectors were analysed as the most critical ones. Particularly, energy efficient and low-carbon measures were evaluated on thermal consumptions, the most important and difficult to solve energy issues in a high-density urban context.

Turin has about 60 thousand heated buildings (232 Mm³), the residential sector is constituted from more than 45 thousand buildings equal to 164 Mm³, while the non-residential buildings are around 14 thousand equal to 68 Mm³ (CTC of Turin, 2015). The residential sector is mainly characterized by large and compact condominiums (surface to volume ratio: S/V ≤ 0.45) and detached houses (S/V ≥ 0.71). Considering the period of construction, Turin has a very old building stock, with 57% of the buildings built before 1960 and more than 80% built before 1970. The residential buildings built in 1970–2000 reach about 15% and after 2006 only 2%.

Turin is also characterized by 40 homogenous urban micro-zones (Figure 3a) reported in Table 2. For each micro-zone, the number of buildings, the heated volumes, the number of inhabitants, the type of building: residential or non-residential (Figure 3b), the main period of construction and the main level of maintenance are reported. Also, the nearest weather station (Figure 3a) was also identified to analyse the energy consumptions taking into account the different microclimate characteristics. As it is possible to observe in Table 2, few micro-zones are characterized by: new buildings built after 1970 (only in the micro-zone n, 39 “Spina 1” there are new buildings) and detached houses (only micro-zones: 22, 23, 24, 25, 26 and 40 near the hill in the Eastern part of the city). Moreover, the buildings are occupied and heated for about the 80% of their gross volume and they are mainly residential buildings with the exception of micro-zones 17, 30 and 38 that are mostly industrial (as reported also in Figure 2b).

Figure 3a.

Localization of the five weather stations in the 40 urban micro-zones of Turin

Figure 3b.

Types of building users (residential, industrial, public and tertiary) analyzed for the districts

Data about energy space heating consumptions are derived from the research projects, Cities On Power (www.citiesonpower.eu), Data4Action (www.data4action.eu) and ENERDATA (www.enerdata.net). Also data about single buildings or blocks of buildings with monthly period were given by the DH Iren company. Data about the energy consumption at municipal scale for the City of Turin were also taken from [26].

The population resident in Turin in 1991, equal to 979,839 inhabitants, fell down to reach the historical minimum in 2002 with 866,134 inhabitants but in the last 10 years the population is approximately constant (±1%). In 2000, Turin registered 879,285 residents and in 2013 the total number of inhabitants of Turin were 872,091 (Figure 4a). Per capita earning from 2000 to 2007/2008 has been growing, but after 2008, it has decreased slightly due to the economic crisis (see Figure 4b, 1,000 EUR/cap from demographic statistics, ISTAT: http://dati.istat.it/). Also per capita income and other indicators have been analysed with the results of the research projects, Odyssee-Mure (www.indicators.odyssee-mure.eu) and ENERDATA (www.enerdata.net).

Figure 4a.

Electricity and thermal energy consumptions (MWh/y) for the years 2000–2013 considering number of inhabitants

Figure 4b.

Electricity and thermal energy consumptions (MWh/y) for the years 2000–2013 considering HDD at 20 °C

Energy sources and greenhouse gas emissions

Considering the different sectors, energy consumptions data for the city of Turin show that, the residential sector (Figure 5a in red, 59%) and the tertiary sector (Figure 5a in blue, 18%) have a major impact on total consumptions, the transport sector is decreasing and the agricultural sector is irrelevant. Relating to greenhouse emissions, the residential sector affects for 52% of total emissions and the tertiary sector affects for 25%, overall, the trend of emissions follows the trend of energy consumption (Figure 5b).

Figure 5a.

The trend of thermal consumptions considering the different sectors: residential, tertiary, transport, agricultural

Figure 5b.

The trend of greenhouse gas emissions due to thermal energy consumptions considering the different sectors

About energy sources, natural gas represents 56% of the energy resources, in particular, the residential sector and the tertiary sector are responsible for 79% of natural gas consumptions (Figure 6a). As shown in Figure 6b, after 2010 natural gas decreased due to microclimatic conditions and the increase of renewable sources. DH is steadily increasing and represents 15% of the energy sources (residential and tertiary are responsible for 20% of DH). Diesel, gas oil, petrol and fuel oil are decreasing. LPG has a non-uniform and slightly increasing trend. Solar, biomass and biofuels (renewable resources) constitute a small proportion of total consumption in 2013, but solar-thermal data shows that from 2000 to 2013 there has been a constant increase with a peak in 2011.

Figure 6a.

The trend of thermal consumption distinguishing by energy sources and type of user

Figure 6b.

Trend percentage of thermal consumption distinguishing by energy sources

Taking into account the residential sector and the tertiary sector, the energy resilience of Turin has been evaluated, as they affect for 79% of Turin’s thermal consumption. The results show that heat energy consumption depends on HDD but also on per capita earning, as shown in Figures 7a and 7b, in fact for the year 2010 the thermal consumption was higher because of cold temperature (higher HHD). In Figure 7b, thermal consumptions have been normalized with 2,620 HDD and, after the 2007, an average decrease of 3% can be observed on normalized thermal energy consumption.

Figure 7a.

Thermal energy consumptions (MWh/y) considering the HDD for the years 2000-2013

Figure 7b.

Thermal energy consumptions (MWh/y) normalized on 2,620 HDD considering earning per capita

A similar trend can be observed on natural gas consumption, as natural gas is the most used fuel in Turin with an average percentage of 78% (2000-2013), while the 20% of the thermal energy is distributed by the DH network (Figure 8a). Every year, natural gas decreases by 3% while, in the last years, the DH network is expanding and then thermal energy distributed by DH is increasing by 8% (Figure 8a). Gas oil, LPG and all other fossil fuels are declining but they are negligible compared to natural gas and DH.

Electricity consumption depends on population but also on per capita income. As reported in literature, the exponential trend of per capita electricity consumption is a function of per capita income, energy price of the electricity and by the saturation impedance of electricity service (red dotted line in Figure 8b) [27].

Figure 8a.

The trend of thermal consumption (with HDD normalization) distinguishing by natural gas and DH for residential & tertiary buildings

Figure 8b.

The trend of electricity consumption normalized by per capita income for residential & tertiary buildings

As shows Figure 9a, greenhouse gas emissions are generally increasing but observing separately, the thermal ones are fairly constant if normalized with HDD and income, while electrical ones increase. In particular, thermal emissions depend on HDD and per capita earning or income, while electric emissions mainly depend on per capita income and on the electricity market (in Turin the population does not vary, ±1%).

Figure 9a.

Thermal and electrical emissions considering HHD and per capita income

Four future scenarios on DH expansion and on solar thermal collectors use have been assumed (Figure 9b): the first in 2030 considers the connection of the existing DH network to all “connectable” residential/tertiary buildings (35,679,069 m³), the second in 2050 with the expansion of the DH network to all the city of Turin with the connection of other 12,802,847 m³ of residential/tertiary buildings (see Table 2). Due to urban and technical/economic limits [20], 35% of residential/tertiary buildings cannot be connected to the DH network. In this work, the considered limits for the city of Turin are: the presence of territorial barriers as rivers and hills, the historic center of the city, the autonomous heating system installations and the buildings with a volume < 2,500 m³.

Figure 9b.

Future scenarios on evolution of TLR and solar thermal technologies in medium (2030) and long-term (2050)

The last two scenarios consider also a further use of +20% of consumption with solar technologies in 2030 and +30% in 2050. Finally, with this last scenario, greenhouse emissions will be reduced of 27% in the medium-term and of 39% in the long-term.

Results on Energy Security Index

In the last part of this work, the energy risk was assessed through the Energy Security Index (for the years 2000–2013, using Odyssee database for Italy). In particular, the method of aggregation of the normalized indicators was used to create the International Energy Security Index [28], an annual energy risk indicator used to identify the policies and other factors that contribute positively or negatively to energy security.

Two indexes for all sectors (residential, municipal, tertiary, industrial) and for residential sector have been calculated considering the following four indicators based on energy security and environmental sustainability:

• Energy security:

  • ○

    The importance of energy to individuals measured by energy consumption per capita (or per dwelling) – 1;

  • ○

    The importance of energy to economic growth measured by primary energy per GDP – 2;

• Environmental sustainability:

• ○

  The importance of policies and economy on greenhouse gas emissions measured by CO2 emissions annual trend – 3;

• ○

  The join effect of consumption per capita and carbon intensity measured by CO2 emissions per capita (or per dwelling) – 4.

All annual data of energy consumption, primary energy and GHG emissions were normalized with climatic corrections.

As mentioned previously, the different indicators have been normalized with the “Min-Max” method, equal weights were assigned to all indicators and, to evaluate the Energy Security Index, the additive aggregation method has been used.

In Figures 10a and 10b, the Energy Security Index was calculated for all sectors, while Figures 11a and 11b report data only for residential sector. The Energy Security Index is decreasing, for both analyses, mainly because all energy consumptions and the GHG emissions are decreasing, also due to the economic crisis (after 2008). This trend was confirmed by analyses on other OECD countries with an increase of the Energy Security Index up to 2008 and then a decrease [28].

Figure 10a.

Indicators of energy security and environmental sustainability for all sectors

Figure 10b.

Energy Security Index for all sectors

Figure 11a.

Indicators of energy security and environmental sustainability for residential sector

Figure 11b.

Energy Security Index for residential sectors: (I1 + I2 + I3 + I4)/4

The energy sector could be a good opportunity to give a boost to the economy also towards sustainability. In Italy, the energy sector needs to focus on more reduction of energy consumption boosting the available renewable energy sources, in particular, solar technologies (available also in urban spaces). Also efficient systems with low environmental impact, as DH, could be promoted in big cities together with smart urban planning to reduce energy consumption, CO₂ emissions with a mitigation of Urban Heat Island effects for a more sustainable and livable urban environment.

This analysis is based on a GIS tool, allowing one to associate the buildings information, from different sources and information levels, with a multivariate analysis, interpreting socio-economic data, technical building parameters (e.g. surface to volume ratio, period of construction, percentage of heated volume and energy demand) and the real characteristics of urban environments.

The procedure evaluated the energy resilience considering the ability of energy systems to absorb and recover from the impact of disruptive events without fundamental changes in function or structure. The method can be applied to other urban contexts, allowing a representation of the energy resilience taking account the energy risk, the sustainability environment, the socio-economic variables, the climatic conditions and the availability of different energy sources for a more sustainable city.

This study could be useful for identifying policies and practices to improve environmental and energy sustainability, for example, the results of consumption data showed that thermal and electrical consumption data depend on per capita income/earning then energy price could play an important role on energy security.

Indicators are useful tools for communicating energy issues related to sustainable development to policy-makers and to measure sustainable development strategies.

Energy security is difficult to measure using a simple indicator. If the number of indicators is small, the energy security index is generally very sensitive to changes in any of the indicators, conversely when number of indicators used is large. This study could be a starting point for future research by implementing the energy security index balancing stability and sensitivity of the index.

The methodology proposed in this study is influenced by the availability of data at urban scale. One of the main limitations in the use of indicators to quantify the energy risk is the need of data. Further research may be conducted to develop indexes that are robust against incomplete information and missing data.

Energy efficiency is essential for improving climate change. DH represents an opportunity for rational use of energy resources and local pollution control in critical urban contexts with high concentration of energy demand but it should be associated to energy consumption reduction and a larger use of renewable energy sources.