Beyond an energy focus, crosscutting studies addressed the need for smarter energy systems while undertaking key issues of water and environment systems. Duić et al. [14] developed the RenewIslands method to integrate energy and resource flows with the aim of providing guidance for sustainable communities. The method was applied to analyse the local needs for commodities, such as energy and water, the available resources, and relevant technologies. Solutions were obtained for the islands of Corvo, Porto Santo and Mljet [14].

In a related aspect, Krajačić et al. [15] applied the energy planning tool of H2RES to the islands of Mljet, Porto Santo, Terceira, and Malta. The aim was to increase the integration of renewable sources and hydrogen into island energy systems. Medic et al. [16] analysed the island of Hvar from the aspect of self-sufficiency based on renewable sources and energy storage systems. Segurado et al. [17] analysed the case of the island of São Vicente in Cape Verde. The use of wind power was proposed as an integrated solution to provide for local water needs based on desalination as well as electrical energy with a penetration well above 25% of the electricity supply by the year 2020. Such cases indicate that smarter energy, water, and environment systems can co-exist to support future oriented scenarios.

The energy infrastructure of cities may be placed in a broader context to jointly assess the sustainability of energy and environment systems. In this respect, Afgan et al. [18] proposed the use of indicators, such as CO₂ emissions and waste, in the process of assessing energy systems. In the related aspect of urban metabolism, Ravalde et al. [19] identified the need to use grey-box metrics to provide insight on specific conversion processes. Related metrics were used to compare the dependencies between energy, water, and waste flows in three cities, namely Beijing, London, and São Paulo. Sun et al. [20] identified key factors that affected the metabolism of the industrial city of Shenyang in China. Emergy units were used to bring energy, waste, and material flows on a common basis to formulate the new metric of emergy per capita [20]. Xia et al. [21] identified seven indicators based on emergy to assess the metabolic structure, density, intensity, and efficiency of Xiamen city.

Chen et al. [22] analysed the sectoral energy flows in Beijing based on ecological network analysis. In a subsequent study, the authors analysed the same city from the aspect of energy-related water and water-demanded energy [23]. In addition, the authors reviewed the scope of urban metabolism studies, the most frequent of which was focused on energy [24]. Tseng et al. [25] used a network process to assess the flow of materials or energy between the nodes of energy, waste, food, agriculture, and domestic sectors in Taipei City.

Related studies that provide an expanded scope for assessing resource flows include Kennedy et al. [26] who proposed a set of indicators to assess urban metabolism in megacities. The indicators included those on energy usage, water consumption, wastewater, and waste alongside urbanized area, climate, and population [26]. Mostafavi et al. [27] developed an analysis tool for urban metabolism to assess the impact of building location and clustering on energy, water and material flows with a geographical dimension. Conke et al. [28] evaluated the changes in material and energy use in the city of Curitiba, Brazil over a timeframe of 10 years up to 2010, including food, energy, and water consumption.

Venkatesh et al. [29] analysed the urban water and wastewater systems of Oslo based on metrics to assess the environmental impacts of resource flows. Behzadian et al. [30] quantified resource flows in the urban water system, including water supply, stormwater and wastewater, and the relative environmental impacts on GHG emissions, acidification, and eutrophication for a northern European city. Zhang [31] reviewed the evolution of studies to assess production, consumption, and circulation within and among components of urban systems. A trend towards a network process to assess urban metabolism was found.

The process of assessing smarter energy systems with the inclusion of environmental indicators can provide enhanced insights on sustainability. Yet there is a gap in the literature for synthesizing the analysis of specific targets for related energy concepts, such as net-zero districts, and indicators that extend beyond energy into water, waste, and CO₂ emissions. In particular, studies that combine an analysis of concepts that can support the transition to both smarter energy systems and environment systems are still limited in the literature.

Concepts based on net-zero targets can allow districts as well as cities to be the hubs for smarter energy systems. The conventional roles of urban areas as consumers of energy can be transformed with diversified local energy production based on renewable energy sources at or above the amount of annual energy usage. In contrast, studies that assess districts with net-zero targets in the context of environmental indicators are still missing in the literature.

The research work aims to expand the contextual framework for analysing net-zero targets within more sustainable energy, water, and environment systems. In particular, the performance of a district with an innovative net-zero target [32]-[33] is compared through per capita indicators for urban metabolism. In this way, the research work puts forth and applies an approach that has relevance to assess three objectives: the more rational use of energy based on energy quality, aspects of water, waste, and CO₂ emissions, and net-zero targets.

The analysis of the existing case study of the city of Uppsala is used as the reference to compare proposals for the new district of Östra Sala backe. The planned district has ambitions to be Uppsala’s most climate friendly district with sustainable solutions [36]. For the analysis of the present case of the city of Uppsala, the available statistics were used to construct Sankey diagrams of the energy, water, and waste flows of the city. Where possible, per capita statistics that were collected by city officials were utilized [37]. In other instances, values were obtained based on calculations from available data.

Proposals for the first two phases of the new district of Östra Sala backe were put forth in [32], [33] with the aim of allowing the district to reach an original net-zero target based on the quality of energy. In this research work, these proposals are subjugated to a comprehensive assessment. Levels of improvement between phases one and two of the Östra Sala backe district are compared to the existing values of the basic flows within the city of Uppsala. Initiatives for Östra Sala backe to improve the flows of water and waste are placed in the context of future scenarios with a 10 year time horizon.

In the prior research work of the author [32], [33], the Net-Zero Exergy District (NZEXD) target was put forth to enable an innovative energy concept for Östra Sala backe. According to the NZEXD target, a district should produce as much energy as consumed on an annual basis at the same grade and quality [32], [38]. The grade and quality of energy is measured as exergy, which is the useful work potential of a given amount of energy based on a Carnot cycle that is defined according to a reference environment temperature.

Eq. (1) provides the formulation of the NZEXD target. Here, the term AEXC is the Annual Exergy Consumption of the district. The term ε_on is the exergy that is produced from renewables within the confines of the district on an annual basis. The summation expresses the sum across all energy related units i = 1 to x in a district for all time increments k = 1 to k = m in a year. The energy related units include buildings, industry, and transportation units:

Eqs. (2, 3) provide the expanded version of the two main variables in the NZEXD target. CF_e and CF_t are the Carnot Factors by which the energy sums P_i(e) for electrical energy and P_i(t) for thermal energy are multiplied. The sum of the related products on the demand and supply sides are used to obtain AEXC [eq. (2)] and ε_on [eq. (3)]:

In the NZEXD target, since the aim is to obtain a net-zero status, inefficiencies must be taken into account within AEXC so that the boundary conditions between the supply and demand sides are unified. Moreover, electrical energy has a CF_e of 1 since all electrical energy can be converted into mechanical energy by common convention [39]. In contrast, the CF_t for thermal energy is calculated from eq. (4). Here, T_ref is the temperature value of the reference environment that is the basis of the exergy definition, such as the ground temperature that closely resembles the average of the outdoor air temperature on an annual basis [40]. T_ex is the temperature of the energy demand or the energy resource supply for which an exergy value is to be calculated. All values are to be expressed in Kelvin (K):

To give an example, the values of CF are 0.86 for natural gas and 0.76 for biogas per 1 kWh of energy [32] based on the adiabatic flame temperatures and the reference ground temperature of Uppsala, which is 279 K [41]. For the waste heat from CHP, T_ex can be 325 K based on the return temperature in the network with the qualities of being a 4GDH.

The NZEXD target can be supported by the principle of distributing the sources of exergy supply to matching levels of exergy demand. In this principle, higher exergy sources are matched with demands that require high exergy (e.g. energy resources with higher values of CF). Similarly, lower exergy sources are matched with demands that can use low exergy (e.g. energy resources with higher values of CF). Hence, the value of AEXC in eq. (2) may be reduced based on lower energy values P_i(e) and P_i(t). AEXC may further be reduced based on a greater share of lower exergy resources in the energy mix with a lower average value of CF. In addition, sources of energy that are often wasted may be better integrated into the supply side to increase the term ε_on in eq. (3).

Based on these principles, the NZEXD target is expected to be influential in reducing the values of energy and exergy per capita in the districts to which it is applied. Figure 1 overviews the contextual framework of the NZEXD target with further extensions to aspects of water and waste. Accordingly, this research work seeks to integrate analyses for the NZEXD target based on the quality of energy with those for urban metabolism. The subsequent sections describe the means of analysing related aspects for comparison.

Figure 1.

Contextual framework for energy, exergy, water, and waste

Initially, the flows of energy, water, and waste were put forth for the city of Uppsala. In so doing, the distribution of energy resources on the supply side and the points of delivered energy on the demand side are linked in an energy supply chain perspective. Eq. (5) provides the summation of these flows that are related by various efficiencies η, namely those for conversion (subscript c), transmission (tr), and distribution (d). The summations of energy E are provided for energy sources (S) and delivered energy (D). Eq. (5) may also be used for CO₂ emissions based on the application of CO₂ factors:

In addition to the flows of energy, eq. (6) represents the flows of water G from the total amount of water that is produced in the city’s waterworks (subscript P). In eq. (6), the total amount is based on the summation of the delivered water to various sectors (O), water leakages (L), the use of water to enable energy conversion processes (E), and other internal uses (I). Hence, eq. (6) further represents an aspect of the energy-water nexus:

The flows of waste W are analysed based on eq. (7) in which the waste that is produced (subscript P) is taken as the sum of the waste that is reused (R) or landfilled (N). Especially in the present case of Uppsala, waste is also a source for local energy production:

Eqs. (5–7) enable the attainment of key values for comparison, such as energy production per capita. Moreover, the identification of the energy flows from primary energy to end use based on Sankey diagrams enables a process of tracing the appropriate values to represent the quality of energy usage in Uppsala, which was unknown. Such Sankey diagrams were also useful to put forth the present state of energy and water usage. Codes that represent 38 entries were written and inserted into a Sankey interface [42].

Due to differences in scale between the city and the district, per capita values for all indicators were used as the basis of comparison for the two related case studies. Eq. (8) is used to compare the changes in the values of a set of urban metabolism indicators between the Östra Sala backe district that is seeking to near the NZEXD target and the values of the city of Uppsala. In eq. (8), the variable M is a given per capita indicator for urban metabolism as selected for the contextual framework of the NZEXD target. Here, subscripts Up and Ös represent Uppsala and Östra Sala backe, respectively:

In cases where scenarios are constructed based on percentage point improvements over the existing status of Uppsala for water and waste, terms M for Östra Sala backe are found from eq. (9). The term V represents the percentage improvement over 100, which may be added or subtracted from 1 based on the specific context of the indicator. In instances where V is added, negative values in eq. (9) will indicate an improvement:

Figure 2 indicates the extended framework for applying the main categories of a selected set of urban metabolism indicators to the analysis of the case studies. In addition to energy per capita, a new indicator is included as exergy per capita. The latter defines a metric with specific importance for the NZEXD target. Exergy per capita represents the benefit of using lower exergy resources over higher exergy resources in a particular district. Exergy per capita has not been applied in other studies in the literature.

Figure 2.

NZEXD urban metabolism indicator set including exergy per capita

In Figure 2, water per capita and waste per capita are other key indicators to assess the flows of other natural resources beyond energy. As the fifth indicator in Figure 2, CO₂ emissions per capita represents changes in reducing the CO₂ emissions impact of the district as a whole. The five main categories of indicators and other related indicators can be used to monitor the urban metabolism of districts seeking to reach the NZEXD target. In this study, this set is proposed to be the NZEXD urban metabolism indicator set.

In practise, the NZEXD urban metabolism indicator set can be used to support multiple aspects of planning to capture improvements in per capita values. For example, spatial planning for districts can be combined with energy and exergy planning to improve the values of the per capita indicators for energy and exergy. Environmental planning can be enhanced to reduce per capita water and waste values. The measures that are planned for Östra Sala backe provide examples of integrating such aspects to improve the performance of the district. Overall, the NZEXD target may be integrated with the planning of smart energy systems and sustainable districts that sustain both natural and human capital. Figure 3 depicts this approach involving multiple aspects of planning.

Figure 3.

Integrated approach for multiple aspects of planning

At the same time, behavioural aspects and lifestyles readily impact resource spending and can undermine the ability to capture the complete savings that may be expected or given to be theoretically possible, e.g. the efficiency gap [43]. For this reason, further insight into such issues is vital in anticipating the changes that may need to take place in support of the planning processes. For example, in Uppsala, a partnership that involves the local energy provider was launched as the “One Tonne Life” project to monitor and analyse the resource spending of a household that is equipped with energy efficient technologies, a solar PV panel, and an electric vehicle [44]. The results are being integrated into future projections.

The present energy system of Uppsala is based on a waste incineration plant, a CHP plant, and a heat pump facility that is located at the city’s wastewater works [45]. The waste incineration plant has an installed capacity of 10 MW_e of electricity and 181 MW_t of thermal energy, which includes 11 MW_t for cooling [45]. The waste incineration plant is able to intake about 52 tonnes of Municipal Solid Waste (MSW) per hour [45], [46].

The CHP plant that is based on peat and wood briquettes with a potential supply of 80 tonnes per hour has an installed capacity of 235 MW_t and 120 MW_e [45]. The briquettes are obtained from a plant in Sveg [47] in central Sweden at a distance of about 400 km from Uppsala. In contrast to wood, peat as a form of fossil fuel waste is not considered to be a form of biomass as a renewable energy resource. By the year 2020, the use of peat will be eliminated through a new CHP plant that will intake only local biomass [45]. Pollutants, including NO_x, are reduced based on processes for flue gas cleaning [45]. In the year 2014, 0.09 kg of NO_x was released per delivered MWh of district heating [45].

The heat pump facility that captures heat from the city’s wastewater and directs the thermal energy to the district heating and/or cooling network has a capacity of 69 MW_t, including 24 MW_t of cooling [45]. Gas turbines and boilers are present in the local energy system as a backup for electricity generation and district heating, respectively. Unlike the city of Stockholm, Uppsala does not use seawater heat pumps to support the supply of thermal energy for district cooling due to its more inner land geography.

In the local energy system, the CHP plant is the main source of electricity generation. During the summer, the CHP is not operated and all of the electrical energy is purchased from the electrical grid. The main source of thermal energy generation is the waste incineration plant that operates on an annual basis, including for cooling in the summer.

In total, the district heating network of Uppsala is 460 km in length with a 14 km district cooling network and 7 km network for steam [45]. The CHP-based district heating network meets the heating needs of about 95% of the buildings in Uppsala [48]. A hot water accumulator is integrated into the system to facilitate fluctuations in district demand [45]. Average supply and return temperatures are about 368 K and 338 K respectively [32], [49].

Primary energy supply for local energy production.

Figure 4 provides the realized primary energy supply for local energy production at Uppsala on a monthly basis in 2014. For Figure 4 only, these values exclude the purchase of electrical energy from the grid and fossil fuels for transport. Based on local convention, the energy supply for waste incineration is counted to be 60% biogenic as renewable energy and 40% as fossil fuel, which accounts for sources of plastic waste that is of fossil fuel origin [45]. This value is compliant with the excepted range of shares for heterogeneous mixture of wastes [50].

Figure 4.

Primary energy supply for energy production in Uppsala [GWh/month]

In Figure 4, fossil fuel also includes oil and coal that is used in the backup boilers that partly serve the district heating network [45]. Based on the monthly values, the total primary energy supply for local energy production is summed to be 1,841 GWh.

The greatest contribution comes from biogenic based waste at 726 GWh, which is followed by fossil fuels at 575 GWh. On a monthly basis, the highest and lowest primary energy spending is 249 GWh (January) and 50 GWh (July), respectively [45].

Sankey diagram of the energy system.

Figure 5 provides the Sankey diagram for the energy system of Uppsala. In Figure 5, the energy flows in the city are traced from energy sources and/or electrical energy purchase from the grid to the final end-use of energy. In total, 3,234 GWh of energy supply is required to satisfy the energy needs of Uppsala at 2,843 GWh, including energy for transport. The energy losses are calculated to be 391 GWh and indicate the amount of primary energy that is unavailable for final energy use.

Figure 5.

Sankey diagram for the energy system of Uppsala [GWh]

At the receiving end, district heating has the largest flow based on the magnitude of energy at 1,266 GWh that represents 45% of the total delivered energy in the city. Energy resources with relatively high levels of exergy, including fossil fuels, are used at the origin of this energy flow. Subsequently, however, the quality of the energy flows degrade in temperature levels from combustion towards end-use. As a result, the quality of this energy flow is not as high as those of electricity and transport. The flows from wastewater to heat pumps and biogas indicate aspects of the energy-water nexus.

In Uppsala, the waterworks produce 200 m³ of water per capita of which 165 m³ of water is consumed on the end-user side [56]. This indicates that 35 m³ of water is wasted due to water leakages in pipelines, which is about the same as the share of water leakages in water production for Stockholm at 17% [55]. Based on the population of Uppsala, it is estimated that a total of 29,697 thousand m³ of water was produced and 24,500 thousand m³ of water was consumed at the end use in 2014. In addition, about 5,197 thousand m³ of the water that was produced by the waterworks was wasted as pipeline leakages.

The water infrastructure of Uppsala includes about 620 km of drinking water pipelines and 1,080 km of pipelines for wastewater and stormwater [56]. The water quality of local water bodies is preserved by treating 100% of the used water in the wastewater treatment plant [56]. The local plant is effective in treating traces of most effluents in both stormwater runoff and incoming wastewater [59]. In 2014, the local lake received “excellent” water quality according to the EU Bathing Water Directive [60].

Overall, within the water cycle, Uppsala receives about 517 litres of rainfall per m² of land surface annually [36], which partly feeds the groundwater reservoirs and water bodies that are utilized by the waterworks to supply drinking water to citizens.

In addition, water is used in the energy system to enable local energy conversion processes. This aspect includes the use of water in cooling towers and circulating water in the district energy network. The energy provider of Uppsala indicates that 290 litres of water was used per delivered MWh of energy in the energy plants in 2014 [45], which is equivalent to 290 m³ of water per delivered GWh of energy. This value results in about 434 thousand m³ of water that is used in the local energy production in Uppsala and 614 thousand m³ of water when the purchase of electrical energy from the grid is included. In this scope, the energy-water nexus results in at least 4.1 m³ of water usage per capita.

Sankey diagram of the water system of Uppsala.

The Sankey diagram of Figure 6 puts forth the water flows in Uppsala from the stages of water production to end use by sector. The water use profile of the nearby region indicates that households consume about 62% of the delivered water while businesses and the public sector consume about 10% and 20%, respectively [55]. At the same time, the share of water that is used internally in plants is given to be about 9% [55]. Accordingly, households may receive about 15,207 thousand m³ of water. This is followed by business and the public sector at 2,323 m³ and 4,858 m³, respectively (see Figure 6). In an aspect of the energy-water nexus, water usage for energy processes at 614 thousand m³ is 29% of the total internal water use. Figure 6 is thus useful to compare the flow of water resources for various purposes in Uppsala.

Figure 6.

Sankey diagram for the water system of Uppsala [thousand m³]

The water flows in the city, however, are not limited to those as represented in Figure 6. Used water is sent to the wastewater treatment plant, which partly becomes a source of energy based on heat pumps and biogas as indicated previously within Figure 5.

At the city level of Uppsala, 219 kg of solid waste, which includes plastics, glass, and organic waste, is produced per capita of which 85 kg is sent to the landfill [61]. This indicates that solid waste was produced in an amount of 32,519 tonnes of which 12,682 tonnes was sent to the landfill. As indicated in Figure 7 for the flows of waste in Uppsala, the remaining amount of 19,836 tonnes is split between waste incineration at 14,146 tonnes and material recycling at 5,691 tonnes. The shares of waste incineration and material recycling in total waste production are 44% and 18%, respectively [61].

Figure 7.

Sankey diagram for the waste system of Uppsala [tonnes]

Beyond the flows that are depicted in Figure 7, sludge from the wastewater treatment plant and residue from the heating plant is directed to the waste plant of Uppsala at Hovgårdens [45]. Alongside combustible waste, these flows become a source of energy supply. Here, it is estimated that 17 kg of solid waste or 9 kg of sludge can produce 1 MWh [45]. Based on the data in Figure 7 for waste and Figure 5 for energy, the amount of energy that is converted from 14,146 tonnes of waste was 1,130 GWh. Hence, an average of 12.5 kg of waste produced 1 MWh in the waste-to-energy scheme of Uppsala.

The NZEXD status for the first phase of the pilot district was calculated by the author in [32]. Reference values of energy usage per capita in Uppsala were used as the basis of the planning. Table 2 summarizes the results of phase one based on the total energy and exergy values for each of the eight measures that have been proposed. For ease of referral with eq. (1), Table 2 sorts the inputs by the demand and supply sides.

In Table 2, the measures on the demand side amount to 113 GWh of energy and 54.3 GWh of exergy. The same amount of energy is satisfied by the measures on the supply side while the energy value of the grid electricity at 4.6 GWh is not included in the column for ε_on since it is not part of the exergy that is produced on-site. Accordingly, only 108.4 GWh of energy and 49.7 GWh of exergy are produced locally in phase one. Based on eq. (1), the NZEXD status is 4.6 GWh, which indicates a nearly NZEXD status since the value is close to the target value of being less than or equal to zero.

In the GSHP option, COP is taken to be 3 based on a renewable power input at 4.6 GWh that provides a thermal output of 13.8 GWh. Such an output provides a 21% share in the thermal energy in the DHC loop that is mainly based on the output of the CHP. Based on eq. (4), values of CF_t that result in the differences between energy and exergy values are 0.14 for the CHP and 0.06 for the GSHP [32].

The proposal for phase one provided a foundation to scale-up the best practices into the second phase of the project and further improve the proximity to the NZEXD target. In this respect, the gap of 4.6 GWh in phase one was initially reduced to 3.5 GWh in the proposal for phase two and further reduced to 1.8 GWh as the result of a stepwise approach with four steps that was developed in [33]. The stepwise approach exemplified the process of exergy transition planning for districts that aimed to near the NZEXD target phase by phase.

Table 3 provides the proposal for phase two based on the results of the process of exergy transition planning. According to the results, 88.4 GWh of energy is required to be supplied for phase two on an energy basis. On an exergy basis, 42 GWh of AEXC is consumed in the district and 40.2 GWh of ε_on is produced in the district. The biomass-based CHP provides the most significant contribution to the supply side of the local energy system.

Table 4 provides the per capita values of the total magnitudes of energy and exergy in Tables 2 and 3 for the phases of the project based on the demand side of the measures. The CF values are also given based on the quality of energy that is used in each measure. The use of the DHDWG option lowers the return temperature in the district heating loop.

According to Table 4, there is a decrease in energy and exergy usage per capita in the proposed measures of DHC and DHDWG between phases one and two. In particular, in the first phase, the total value for both of these measures was 13.4 MWh of energy and 1.7 MWh of exergy per capita. In contrast, the extension of the same measures into phase two is based on 10.5 MWh of energy and 1.4 MWh of exergy per capita.

The indicator set for assessing urban metabolism in the context of the NZEXD target was applied to phases one and two of the district of Östra Sala backe. The per capita values of these indicators were compared to those of Uppsala based on eq. (8) using inputs from eqs. (5–7) for energy, water, waste and CO₂ emissions as needed. The results of the urban metabolism indicator set as put forth and analysed herein are presented in Tables 5–10, including respective comparisons. In particular, the results are provided in Table 5 for energy per capita, Tables 6-7 for exergy per capita, Table 8 for water per capita, Table 9 for waste per capita, and Table 10 for CO₂ emissions per capita.

The comparison of the city of Uppsala and the district of Östra Sala backe based on selected urban metabolism indicators in the contextual framework of the NZEXD target provides an original approach in the literature. These indicators have covered aspects of energy, water, waste, and CO₂ emissions as well as exergy. As a whole, this process has enabled the process of benchmarking proposals for the first two phases of the district to the city in which it will be established. As a district that aims to be a pioneering example of greater urban sustainability, the district shall provide improvements over the existing status of the city. Based on Östra Sala backe, the city of Uppsala will be establishing a living laboratory to test more sustainable production and consumption systems.

Future oriented 10 year scenarios for Uppsala.

Figure 8 summarizes the use of the benchmarked data for a two way comparison between Uppsala and Östra Sala backe. Figure 8 includes per capita values for energy and exergy usage, water consumption, waste generation, and energy and wastewater related CO₂ emissions. The first numerical row provides the present values of the per capita indicators for Uppsala. For comparison, the last two columns provide best practice values from the phase two proposal for Östra Sala backe for energy and exergy. The subsequent rows are based on alterations in these values in a 10 year timeframe. Accordingly, four scenarios are represented in Figure 8 such that there is a 1%, 2%, 3% or 4% improvement in the values of the indicators each year as the value of V in eq. (9) for 10 years. As a result, these scenarios represent improvements of 10%, 20%, 30%, and 40% from present values at the end of 10 years.

Figure 8.

Scenarios for the NZEXD urban metabolism indicators

Already, the best practice values from the phase two proposal for Östra Sala backe exceeds one of these scenarios based on annual improvements in the present values of Uppsala. For example, the energy usage per capita at 17.7 MWh in phrase two exceeds the corresponding value in the 2% per annum scenario in which Uppsala would improve energy usage per capita to 18.6 MWh (see shaded area in Figure 8). In contrast, exergy consumption per capita at 8.5 MWh in phase two exceeds the respective value in the more ambitious 4% per annum scenario based on a reduction of 42%. In this scenario, Uppsala would attain exergy consumption per capita at 8.8 MWh.

From the perspective of these scenarios, Östra Sala backe may be considered to be about 10 years ahead of Uppsala given the specified rates of improvement on an annual basis. The strategy of Östra Sala backe in limiting the use of high exergy energy resources in the process of nearing the NZEXD target also represents the leap that is made under the exergy per capita indicator. The energy system configuration involves a greater share of lower exergy resources. At the same time, annual improvements in the indicators are also possible for Östra Sala backe. For this reason, both Uppsala and Östra Sala backe may record further improvements.

Based on the values in Tables 5–10, the envisioned reductions in water consumption and waste generation for Östra Sala backe would enable a performance that meets or exceeds either the 10% or 20% improvements in the 10 years scenario for Uppsala. In the aspect of water consumption, the targeted values at 148 m³ and 132 m³ per capita for phases one and two are better than or equal to the 10% and 20% scenarios, respectively. In the aspect of waste generation, only the targeted value for phase two at 197 kg per capita exceeds the 10% scenario for Uppsala. In contrast, based on the values in Table 10, Östra Sala backe exceeds the values of all scenarios in CO₂ emission per capita.

Figure 9 puts forth the vision for Östra Sala backe in which the district is able to have savings in aspects of energy, exergy, water, waste, and CO₂ emissions when compared to its surrounding urban environment. The left inset provides images from Östra Sala backe [62] with foreseen low energy consuming building designs. The improvement of energy and exergy savings to meet or near the NZEXD is taken to be a key aspect of improving urban metabolism. These savings may be realized based on better exergy matches in the energy system for high and low exergy requiring energy demands, a more efficient water system network, a better waste management system, and increased renewable energy penetration.

Figure 9.

Improving the urban metabolism of NZEXD