تجزیه و تحلیل سیستم انرژی از یک منطقه اکسرژی خالص صفر آزمایشی

The Rational Exergy Management Model (REMM) provides an analytical model to curb primary energy spending and CO2 emissions by means of considering the level of match between the grade/quality of energy resources (exergy) on the supply and demand sides. This model is useful for developing forward-looking concepts with an energy systems perspective. One concept is net-zero exergy districts, which produce as much energy at the same grade or quality as consumed on an annual basis. This paper analyzes the district of Östra Sala backe in Uppsala Municipality in Sweden as a pilot, near net-zero exergy district. The district is planned to host 20,000 people at the end of four phases. The measures that are considered include an extension of the combined heat and power based district heating and cooling network, heat pumps driven on renewable energy, district heating driven white goods, smart home automation, efficient lighting, and bioelectricity driven public transport. A REMM Analysis Tool for net-zero exergy districts is developed and used to analyze 5 scenarios based on a Net-Zero Exergy District Option Index. According to the results, a pilot concept for the first phase of the project is proposed. This integrates a mix of 8 measures considering an annual electricity load of 46.0 GW he and annual thermal load of 67.0 GW ht. The exergy that is produced on-site with renewable energy sources is 49.7 GW h and the annual exergy consumed is 54.3 GW h. The average value of the level of match between the demand and supply of exergy is 0.84. The paper concludes with advice for a more efficient usage of energy resources in the energy systems of net-zero exergy districts of the future.

The redesign of future energy systems must better integrate renewable energy sources and reduce primary energy spending while substantially lowering CO2 emissions. This requires more innovative changes in the energy system, possibly with districts being the agents of change. Studies in the literature focus on research questions related to the design of district heating networks, the maximum integration of renewable energy sources, and the role of energy efficient and/or energy producing buildings within these systems. For the purposes of this paper, this literature is grouped into four headings, which define stages of an energy supply chain leading to the delivery of more sustainable energy services, namely production and conversion, distribution networks, energy storage, and end-usage. The main highlights of the literature under these headings are given below while Fig. 1 presents an overview.1.1. Production and conversion on the supply side On the supply side, Münster et al. compare district heating grid expansion to individual heating and determine that district heating networks should cover about two thirds of heat consumption in Denmark [1]. Lund et al. find a similar share while proposing that there should be a gradual expansion of district heating over individual heat pumps [2]. Finney et al. focus on the expansion of the district heating network for the UK city of Sheffield while considering options for future heat sources [3]. Østergaard et al. propose a 100% renewable energy system for the Danish city of Frederikshavn by integrating low-temperature geothermal energy for district heating with other supply side technologies, such as off-shore wind power, large-scale solar collectors, and local waste incineration [4]. It is seen that primary energy consumption can be sustainably reduced based on changes in the production system even before savings in end-usage [4]. For the Swedish city of Linköping, Wetterlund et al. analyze two options for district heating based on biomass gasification, including the co-production of synthetic natural gas [5]. Chow et al. analyze a district cooling system for a new urban development in Hong Kong based on direct seawater cooling [6]. Haiwen et al. compare an electricity-driven seawater heat pump system and a central boiler system as district heating options for coastal areas in China, such as Dalian [7]. Kwon et al. analyze the concept of a district heating system that is driven by the recovery of waste heat from urban facilities (e.g. water treatment plant, industrial waste heat, subway waste heat) based on a large-scale, two-staged compression heat pump system [8]. 1.2. Distribution networks for the delivery of energy services Other than a focus on supply side technologies that support district heating networks, Pirouti et al. analyze issues related to distribution, such as a variable supply temperature operating strategy [9]. Tol et al. compare pipe dimensioning methods, substation types, and network layouts in low-temperature district heating systems, including looped and branched network layouts [10]. In another study [11], a network layout with connection to low-energy buildings in a new, suburban settlement area in Trekroner, Denmark is proposed with an operating supply temperature of 55 °C (328 K). As a result, these and other studies address the more working level issues of future district heating networks, which are required for their efficient operation. 1.3. Energy storage for enhanced flexibility Another factor of concern for the design of future district heating networks is energy storage. Verda et al. take the Italian district of Turin as a case study to show that thermal storage can increase primary energy savings in district heating networks [12]. This is based on increases in the annual operating hours of combined heat and power units that serve district heating systems, which reduce the usage of individual boilers [12]. Nuytten et al. couple thermal energy storage with a combined heat and power system to increase the flexibility in matching the supply and demand of thermal energy loads in Flanders [13]. Krajačić et al. determine the role of smart energy storage in allowing the energy system of Croatia to become a 100% independent energy system [14]. The scope of smart energy storage includes thermal storage with phase change materials to better facilitate the use of options for low temperature heat generation. 1.4. Improvements in end-usage on the demand side The impact of energy efficient and/or energy producing buildings on future district heating networks have been questioned in several studies. Difs et al. analyze the effect that energy conserving measures in multi-dwelling buildings may have on the district heating system in Linköping [15]. Based on the same city, Åberg et al. determine that the reduced heat demand due to higher energy efficiency in buildings primarily decrease heat-only production [16]. Nielsen et al. expand the focus to include not only energy conserving measures but also excess heat production from future, energy producing buildings that are connected to district heating grids [17]. Based on the case of Frederikshavn, Sperling et al. find that combining end-use energy savings and district heating expansion improves the overall system efficiency [18]. 1.5. Research trend for a more integrated approach Rather than a focus on single technologies and/or specific stages of the energy supply chain, some studies find that a more integrated approach is required to optimize district energy systems. These studies cover a greater span as indicated in the lower part of Fig. 1. For example, in the case of Frederikshavn, more efficient generation technologies were combined with energy-efficient retrofitting, which effectively reduced the energy import of the district [18]. A similar view is adopted in [19], where Østergaard et al. combine a mix of energy generation options with energy savings to propose a 100% renewable energy scenario for the Danish city of Aalborg. These include low-temperature geothermal heat, wind power, biomass, heat storage, electric vehicles, and light-rail [19]. As stated by Lund et al. in [20], alternative propulsion options for transport are a key element for coherent energy system analysis of future scenarios. Weber et al. consider the mix of centralized and distributed technologies, including heat pumps and solar thermal collectors, and their distribution networks for an eco-town located in the South of England [21]. In [22], Sanaei et al. focus on the synthesis of components in energy systems with an effort to find the most energy efficient design for an integrated system for the Yazd district in Iran. In [23], Krajačić et al. propose three electricity production scenarios for a 100% renewable power system in Portugal. Ćosić et al. integrate energy reductions in buildings, the industry, and transport in proposing a 100% renewable energy system for Macedonia [24]. 1.6. Scope of the research work The above sample of the literature indicates a trend towards a need for a more integrated, “energy systems perspective” and more coordinated action that involves the energy supply chain from beginning to end. The scope of this research work focuses on the integration of a district energy supply chain with the aim of bringing a pilot district closer to a “net-zero” status. A net-zero status requires the ability to produce as much energy at the same amount and/or quality as consumed on an annual basis. As a result, the necessity to reduce energy spending and increase renewable energy production within the district becomes a more immediate concern. The pilot district is located in Uppsala, Sweden and the net-zero status is based not only on the amount of energy but also on the grade or quality of energy (exergy). The merit of the research work comes from a unique focus on a net-zero target at the district level and a mix of energy saving measures along with measures to increase local renewable energy production. In addition, a novel analysis tool is developed to assist in the decision-making process to bring the pilot district closer to the net-zero target. This analysis tool integrates an index that is used to evaluate the mix of measures and their levels of penetration. While the above literature emphasized the need for an integrated, energy systems perspective, none of the studies approached this need from a net-zero district perspective, which is applied to a pilot district in this research. As a result, the research satisfies the objectives of analyzing a pilot district based on a net-zero exergy district target and developing an analysis tool. The paper is organized into four main sections starting from an introduction of a model called the Rational Exergy Management Model that is developed by Kılkış in [25] to allow for the better use of energy resources. This is followed by an overview of the pilot district that is in the planning phase in Uppsala, Sweden, and the components of its energy system based on eight measures that will enable it to near a net-zero exergy target. The third section focuses on the new analysis tool and index that is developed in this paper to compare a mix of different measures for the pilot district. The paper includes a discussion of the results that integrate the mix of measures into a coherent whole for a near net-zero exergy district status before concluding with key recommendations in light of future expected developments for net-zero exergy districts.

The net-zero exergy target requires districts to put forth better coordinated action to integrate renewable sources and reduce primary high exergy spending. Such an integrated perspective for the energy system involving measures for the supply chain from beginning to end can bring districts closer to the net-zero exergy target. This paper has shown that the Östra Sala backe district in Uppsala Municipality has the potential to be a pilot, net-zero exergy district given that the various measures of the project are integrated to complement one another, i.e. measures to reduce AEXC and increase on-site exergy production. Such an energy systems perspective has been enhanced with the analysis of measures for the district based on the REMM Model, which has aided in optimizing the proposed energy system for Östra Sala backe for reducing the spending of high exergy resources and compound CO2 emissions. In addition to the pilot district analysis of Östra Sala backe, this paper has further developed the REMM Analysis Tool for net-zero exergy districts. This Tool will be useful to compare, contrast, and recommend case studies for different options to bring other pilot districts closer to net-zero exergy targets. Another vital aspect is the NEDOI index, which has been developed as a means to rank and compare different measures and different scenarios for Östra Sala backe and any other pilot district. According to the results of the scenarios that were considered for Östra Sala backe, measures that have high values of ψRi as well as potential for high levels of penetration should receive priority in bringing districts nearer to the net-zero exergy target. The pilot of Östra Sala backe may be useful in guiding other pilot districts towards an innovative energy system that is optimized to near or meet the net-zero exergy district target. Based on the measures that are integrated in Östra Sala backe, lower exergy demands (heating and cooling) should be matched to lower exergy energy resources (i.e. waste heat or low temperature energy resources) to avoid the waste of higher exergy resources, which have better uses elsewhere in the energy system, such as the generation of electricity for public transport. As a forward-looking concept with an energy systems perspective, the net-zero exergy district target can offer an effective concept to reduce the spending of high exergy resources and to integrate renewable sources while substantially lowering CO2 emissions. Thus, the net-zero exergy target can allow districts to be the change agents of a more sustainable energy system.