Article Waste Energy Recovery from Natural Gas Distribution Network: CELSIUS Project Demonstrator in Genoa

Davide Borelli †, Francesco Devia †, Margherita Marré Brunenghi †, Corrado Schenone *,† and Alessandro Spoladore †

Received: 5 October 2015; Accepted: 14 December 2015; Published: 18 December 2015 Academic Editors: Francesco Asdrubali and Pietro Buzzini

DIME—Dipartimento di Ingegneria Meccanica, Energetica, Gestionale e dei Trasporti, Università degli Studi di Genova, Via All’Opera Pia 15/A, 16145 Genova, Italy; [email protected] (D.B.); [email protected] (F.D.); [email protected] (M.M.B.); [email protected] (A.S.) * Correspondence: [email protected]; Tel./Fax: +39-010-353-2577 † These authors contributed equally to this work.

Abstract: Increasing energy efficiency by the smart recovery of waste energy is the scope of the CELSIUS Project (Combined Efficient Large Scale Integrated Urban Systems). The CELSIUS consortium includes a world-leading partnership of outstanding research, innovation and implementation organizations, and gather competence and excellence from five European cities with complementary baseline positions regarding the sustainable use of energy: Cologne, Genoa, Gothenburg, London, and Rotterdam. Lasting four-years and coordinated by the City of Gothenburg, the project faces with an holistic approach technical, economic, administrative, social, legal and political issues concerning smart district heating and cooling, aiming to establish best practice solutions. This will be done through the implementation of twelve new high-reaching demonstration projects, which cover the most major aspects of innovative urban heating and cooling for a smart city. The Genoa demonstrator was designed in order to recover energy from the pressure drop between the main supply line and the city natural gas network. The potential mechanical energy is converted to electricity by a turboexpander/generator system, which has been integrated in a combined heat and power plant to supply a district heating network. The performed energy analysis assessed natural gas saving and greenhouse gas reduction achieved through the smart systems integration.

Keywords: smart city; district heating and cooling; waste energy recovery; EU strategy for GHG reduction; natural gas network

1. Introduction The European Union (EU) strongly pursues the reduction of greenhouse gas (GHG) emissions in order to implement the Kyoto Protocol commitments, with a final target of 20% per year emissions saved by 2020 [1]. To this purpose a better use of energy in urban areas assumes a crucial role, since they are responsible for almost 80% of total GHG emissions but, at the same time, they play a crucial role in the safeguarding of the environment and in creating more resilient urban conditions [2]. In particular, it is estimated that in European countries almost 40% of energy is consumed in buildings and the 70% of this energy is deployed in heating and conditioning, so defining a leading sector in the effort to get a sustainable use of power resources [3]. EU policies aim for energy efficiency improvement, renewable energy source (RES) usage increase, and GHG emission reduction by 2020 through a large number of programs and regulatory

Sustainability 2015, 7, 16703–16719; doi:10.3390/su71215841 www.mdpi.com/journal/sustainability Sustainability 2015, 7, 16703–16719 actions. Since the beginning of this century the EU has been implementing operating environmental policies to confront climate change scenarios and support low-emissions activities. It is worth noting from the document “A European Strategy for Sustainable, Competitive and Secure Energy” [4], that it anticipated the exigency of a common planning approach to energy efficiency and RES exploitation, or the “Strategic Energy Review” [5], which introduced the well-know 20-20-20 strategy. Moreover, the “Renewable Energy and Climate Change Package” [6], launched the Covenant of Mayors initiative, all the way up to the “Roadmap for Moving to a Low-carbon Economy in 2050” [7] until the “2030 Framework for Climate and Energy Policies” [8],. The EU strategy has been following a general address at international level [9], aiming for an environmental policy for GHG reduction [10] and energy saving [11], especially focusing on a more intense use of innovative technologies [12,13]. Nevertheless, turning general determination into operative policies is not easy; the transition from a principles and objectives statement to implementation of actions may be complex. Indeed, the real impact of improvement actions is not easy to assess, and their effectiveness needs to be carefully analyzed [14]. It is therefore crucial to demonstrate innovative technologies able to facilitate saving actions as well as to promote a deep involvement of citizens, stakeholders, and institutions, beginning with municipalities, the basic unit of public administration in much of the world [15]. The “Smart Cities and Communities” initiative within the 7th Framework Programme moves in this direction, by operatively promoting carbon savings in cities through the efficient use of energy potentials and the effective recovery of energy losses in urban areas. The EU project CELSIUS “Combined Efficient Large Scale Integrated Urban Systems” aims to demonstrate the possibility to boost energy efficiency through the smart integration of urban systems. District heating and cooling (DHC) can be enhanced by a smart waste energy recovery strategy involving different categories of plants and cities. Thus, twelve demonstration projects, covering the most major aspects of innovative urban heating and cooling for a smart city were delivered in Gothenburg, Cologne, London, Rotterdam, and Genoa [16]. In Genoa, the focus is about energy recovery from the pressure drop between the main supply line and the city natural gas network. The potential mechanical energy is converted into electricity by a turboexpander/generator system, which has been integrated in a combined heat and power (CHP) plant in order to supply a district heating network. This smart integration of infrastructures makes a large energy saving, with a recovery of up to 2.9 MWh/year of electricity by the turboexpander 3 and 500,000 Sm /year of NG saved overall, and with a corresponding reduction of CO2 emissions of over 1000 tons per year. Project implementation, demonstrator performance, and outcome analysis are reported in the present paper, together with time domain results for the energy network, aiming to communicate the innovative solution implemented.

2. CELSIUS Project

2.1. Project Goals, Partnership and Roadmap The CELSIUS project is a district heating-cooling demonstration and transfer knowledge project which focuses on increased excellence in smart and integrated urban energy systems. It is a co-funded project by European Union’s Seventh Framework Programme for research, technological development and demonstration (grant agreement No. 314441) in the framework of smart cities projects. It supports the achievement of EU’s 20-20-20 goals by helping cities in addressing energy resilience and security, energy poverty as well as reducing GHG emissions. By creating resilient low carbon energy system, Celsius cities will be able to support energy needs and demand for citizens as well as contribute to mitigate climate change. The main partners consist of five city groups, with a combination of municipalities, research centers and utility companies for each one. This combined groups represent a key factor as they gather together the main actors involved in heating and cooling infrastructures and systems. CELSIUS cities are the City of Gothenburg, City of Rotterdam, Cologne City, Comune di Genova and London City

16704 Sustainability 2015, 7, 16703–16719 also including the Islington Council. They have already a proven track-record as well as a strong commitment to increase energy efficiency. All of them are already members of the Covenant of Mayors, the mainstream European movement involving local and regional authorities, voluntarily committing to increasing energy efficiency and use of renewable energy sources in their territories. Research centers are very well represented with members from Imperial College in London, London School of Economics, Technical Research Institute of Sweden, Interactive Institute Ab, Technische Universteit Delft, Fachhochschule Koln, and University of Genoa with specific excellence function as within system integration, production, storage etc. with which they support the demonstrators. The complementary utility companies are Gothenburg Energy for Gothenburg, RheinEnergie for Cologne, UK Power Net for London, Warmtebedrijf Exploitatie and Noun for Rotterdam, and Genova Reti Gas for Genova. They are a strong and committed group who will take major investments in this project as of 55 millions of € and have collectively previously spent hundreds of millions of euros. For CELSIUS these utility companies will build twelve new demonstrators and as well use more than twenty already operational demonstrators to be able to present the full CELSIUS City concept. An important part of the CELSIUS demonstration projects is devoted to illustrate how cities can become more energy efficient by using the potential of DHC systems to capture and exploit the wide-range of waste heat sources—heat being produced within the city and not being used but just discharged into the local environment—that are available within cities [17]. The CELSIUS project intends also to help cities to understand and embrace the role that the DHC system can play in a city’s future energy system. Moreover, in addition to technical demonstration projects, CELSIUS assesses financial, political, and social barriers to DHC networks, trying to develop solutions to overcome them and developing new business models, in order to both value resource conservation and encourage cities to develop more self-sufficient energy systems. To this extent, as part of this wider engagement, the CELSIUS project aims to recruit fifty cities from across the EU that are interested in the role the DHC system can play. These will form a network of “CELSIUS cities” and the CELSIUS project will use its extensive range of knowledge and expertise—namely “CELSIUS toolbox”—in all areas of intelligent DHC networks to help them better understand how the DHC system can contribute to develop secure, affordable, and low carbon energy systems in their city [18,19]. To this extent, a part of the project would be dedicated to create an expert team, constituted of a mix of utility company managers, technical expertise, and senior personnel. This group of selected people will be involved from time to time in the support of the new city and will provide the necessary know how, depending on the needs of the city itself. This contribution could range from the first steps a city needs to take, in understanding the role that the DHC system could play, to gaining political support within the city for it. Moreover, it can vary from developing a strategic position to help towards planning, developing, specifying, constructing, extending, and/or optimizing intelligent DHC systems within the selected cities. To begin with, a selected group of seven cities (Athens, Riga, Ghent, Gdynia, Warsaw, Gdansk, and Villadecans)—the Replication Cities (RC)—has been involved in the project. These cities are not directly partners of the project, but have an interest to replicate CELSIUS solutions fitting specifically to their needs, characteristics and city plans. They have already committed themselves via a Letter of Support to be test pilots for the CELSIUS Roadmap. The idea is to create a complete and general “vision”of a Celsius City, not only to provide just a technical toolbox [20]. A roadmap that can summarize all the learning, expertise and technologies gained by the five partner cities into an approach that can be used to support large-scale deployment of energy infrastructure projects across the EU. This vision, spread out as much as possible, will lead to create political acceptance of the important role intelligent DHC system have to play in the EU’s present and future energy systems and to lobby for a supportive policy, legislative and regulatory framework.

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2.2. Demonstrators There is ever increasing urgency for Europe to make the transformation to a low-carbon and resource efficient economy to meet climate and energy security targets and, more importantly, to avoid catastrophic climate change. Nowadays, almost 50% of the total energy consumed in the EU is used for the generation of heat for either residential or industrial purposes and the majority of this energy is produced using fossil fuels. With few exceptions, cooling is, achieved through processes driven either by electricity, which is still mainly produced from fossil fuels, or waste heat, where generated and is often not even captured or used for meeting heat demand elsewhere. Several key challenges obstruct Europe’s cities to become the resource efficient and low carbon cities they are required to be in the global economy of the 21st Century. First of all, many European cities show a lack of thermal grid coverage. To reverse this trend, it is needed the development of new ground-breaking and financially competitive thermal grids; state-of-the-art solutions have already been implemented in some urban areas, but the lessons learnt and experiences need to be shared more widely. At present, new efficient thermal networks tend to be limited to small areas as this is the easier way in a city to start the building of district heating. Another important challenge is represented by the huge amount of waste or secondary energy generated but not captured and utilized for heating purposes nowadays in the EU. Waste heat produced in the EU would actually be sufficient for heating the EU’s entire building stock. At the same time, there is just not the heating distribution grid available to transfer it to where it is demanded and thus it can be used. Therefore, it is currently necessary to demonstrate and develop innovative technologies able of raising the recovery of waste energy. In addition, it is crucial to define, implement, and test business models that evaluate an agreed fair price, for example in respect to industrial waste heat providers, for delivering waste heat to a network or a final user. In this sense, it is fruitful to capitalize existing experience and ongoing research activity, in particular regarding Energy Networks Integration and Urban Energy Hubs. Celsius demonstrators just operate in this direction, opening to on-field application of concepts as Distributed Energy Resources optimization [21] and Urban Energy Hub [22]. This last approach analyzes the relations between input and output energy flows and thus can be used to reduce energy consumption and optimize waste energy recovery. The energy hub concept includes decentralized and local energy technologies such as photovoltaic, biomass, or small hydro power, together with district heating systems, building and district conversion, and storage technologies at neighborhood level [23]. These methods, as well as simulation tools for district scale energy systems [24], can be therefore capitalized and applied in order to increase the efficiency of urban energy systems. In order to use energy in the most sustainable way, it seems of crucial importance to reduce peak loads and avoid unnecessary energy supply from fossil fuel driven processes. As the construction and industrial sectors develop more energy efficient solutions, this will start to skew energy demand away from heat, increasingly redirecting it towards cooling. In addition, more energy efficient buildings will also have more dynamic energy demand profiles that will require intelligent storage and load control in combination with energy management systems and increased awareness of the end user. The solution seems to be intelligent process integration built on flexible systems that are ready for a dynamic world using less energy, so that the energy supplied can ultimately come from energy efficient and renewable energy technologies, such as CHP [25]. One of the main goals of CELSIUS project is to demonstrate a cost-effective and energy efficient heating and cooling system for replication and massive role, while ensuring a healthy urban environment. Nonethless recovering waste energy in an efficient and smart way seems crucial like favouring the integration of renewable energy. A possible goal seems a total primary energy reduction of 4500 TWh/year, and a reduction of 900 MMTCD (Million Metric Tonnes of Carbon Dioxide) per year.

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The CELSIUS project will show the range of opportunities that exist for using DHC networks to maximize the efficient use of primary and secondary energy sources in a city: this will be done through a series of related and complementary demonstration projects. CELSIUS intends to play a leading role in allowing cities to make a cost effective transition to a low carbon and resource efficient economically competitive city. The CELSIUS project will have twelve new demonstration projects (see Table1) and, in addition, twenty existing ones in Cologne, Genoa, Gothenburg, London, and Rotterdam. These cities will evaluate and apply the technical, social, and economic feasibility of the CELSIUS vision and concept and will use the experience and learning to compile a competitive and energy efficient CELSIUS urban heating and cooling concept. The aim is that the product of the CELSIUS project, a toolbox, will then be able to support any city in Europe to maximize the utilization of its primary and secondary energy resources in an integrated way that minimizes its operational costs and carbon emissions whilst maximizing its energy efficiency [26].

Table 1. Former CELSIUS new demonstrators list.

Name Code City Using buildings as short term storage GO1 Gothenburg District heating to white goods GO2 Gothenburg District heating to ships GO3 Gothenburg The Heat Hub RO1 Rotterdam Industrial Ecology RO2 Rotterdam Connection of existing industries RO3 Rotterdam Integrating cooling solutions RO4 Rotterdam Active Network Management and Demand Response LO1 London Capture of two sources of waste heat and the integration of a thermal store LO2 London The extension of the Bunhill “seed” network LO3 London Sewage Water CO1 Cologne Energy recovery from the natural gas distribution network GE1 Genoa

Through a collaboration with key-actors across the value chain—cities, utilities, industry, innovative and energy-efficient technology suppliers, politicians, and financiers and the leading European universities and research centers—CELSIUS will assure that the targets for the demonstrators will be achieved. They will in fact contribute to developing the CELSIUS toolbox and will be physically available for new replication cities in each stage of the implementation.

3. The Genoa’s Demonstrator The district of Genoa Gavette is located in the north of the city (Latitude: 44.435|Longitude: 8.959). The area includes different industrial activities, residential buildings and offices. From the energy point of view, the whole district requires a high amount of heat and a discrete electrical demand throughout the year (Figure1). The high concentration of energy demand, as is described in the following paragraphs, provides the economic basis for a possible innovative improvement of production systems. Major area’s industries concern the distribution of natural gas, accomplished by two different activities: through a central expander, for the distribution through the town piping network, and through a compressor station for the refueling of vehicles. Additional needs come from the electrical and thermal demands to supply a central fire department, a school, mechanical workshops, dressing rooms, and offices of companies operating in the nearby areas such as public transport and distribution of domestic water and gas.

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Table 1. Former CELSIUS new demonstrators list.

Name Code City Using buildings as short term storage GO1 Gothenburg District heating to white goods GO2 Gothenburg District heating to ships GO3 Gothenburg The Heat Hub RO1 Rotterdam Industrial Ecology RO2 Rotterdam Connection of existing industries RO3 Rotterdam Integrating cooling solutions RO4 Rotterdam Active Network Management and Demand Response LO1 London Capture of two sources of waste heat and the integration of a thermal store LO2 London The extension of the Bunhill “seed” network LO3 London Sewage Water CO1 Cologne Energy recovery from the natural gas distribution network GE1 Genoa

The CELSIUS project will have twelve new demonstration projects (see Table 1) and, in addition, twenty existing ones in Cologne, Genoa, Gothenburg, London, and Rotterdam. These cities will evaluate and apply the technical, social, and economic feasibility of the CELSIUS vision and concept and will use the experience and learning to compile a competitive and energy efficient CELSIUS urban heating and cooling concept. The aim is that the product of the CELSIUS project, a toolbox, will then be able to support any city in Europe to maximize the utilization of its primary and secondary energy resources in an integrated way that minimizes its operational costs and carbon emissions whilst maximizing its energy efficiency [26]. Through a collaboration with key-actors across the value chain—cities, utilities, industry, innovative and energy-efficient technology suppliers, politicians, and financiers and the leading European universities and research centers—CELSIUS will assure that the targets for the demonstrators will be achieved. They will in fact contribute to developing the CELSIUS toolbox and will be physically available for new replication cities in each stage of the implementation.

3. The Genoa’s Demonstrator The district of Genoa Gavette is located in the north of the city (Latitude: 44.435|Longitude: 8.959). The area includes different industrial activities, residential buildings and offices. From the energy point of view, the whole district requires a high amount of heat and a discrete electrical Sustainability 2015, demand7, 16703–16719 throughout the year (Figure 1). The high concentration of energy demand, as is described in the following paragraphs, provides the economic basis for a possible innovative improvement of production systems.

Figure 1. Aerial representation of the Gavette district. Sustainability 2015, 7, page–pageFigure 1. Aerial representation of the Gavette district. 5 3.1. The Local EnergyMajor area’s Demand industries and concern Waste Energythe distribution Potential of natural gas, accomplished by two different activities: through a central expander, for the distribution through the town piping network, and The localthrough energy a compressor need ofstation the for district the refueling is now of providedvehicles. by the natural gas station. The use of gas in Genoa is closelyAdditional linked needs to thecome building from the heating’selectrical an sector,d thermal as theredemands is a to prevalent supply a usecentral of gasfire instead of department, a school, mechanical workshops, dressing rooms, and offices of companies operating in gasoline orthe other nearby fossil areas fuels.such as The public industrial transport and use distribution is rather limitedof domestic since water there and isgas. only small activity with small energy demands. Thus, gas volumes processed by the plant (Figure2) have a strong seasonal variation closely3.1. The Local related Energy to Demand the periods and Waste when Energy the Potential heating systems are turned on (according to Genoa heating regulation,The local from energy 1 November need of the district to 15 is April). now provided by the natural gas station. The use of gas in Genoa is closely linked to the building heating’s sector, as there is a prevalent use of gas instead of The natural gas is captured from the national transport network at a pressure of 24 barg; it is gasoline or other fossil fuels. The industrial use is rather limited since there is only small activity with then processed in order to reduce the pressure to 5 bar necessary for its distribution at the city level. small energy demands. Thus, gas volumes processed byg the plant (Figure 2) have a strong seasonal The expansionvariation closely process related isto the actually periods developedwhen the heating using systems throttling are turned valves, on (according in according to Genoa with the Joule-Thompsonheating regulation, principle, from in 1 November order for to this15 April). system to accomplish an isenthalpic transformation dissipating theThe pressure natural gas energy is captured contained from the national in the transport fluid. Thenetwork expansion at a pressure process of 24 bar wouldg; it is then result in an processed in order to reduce the pressure to 5 barg necessary for its distribution at the city level. excessive reduction of the temperature of the expanded gas affecting the safety of the plant due to The expansion process is actually developed using throttling valves, in according with the Joule- the risk ofThompson freezing principle, of the moving in order for parts this system and to to theaccomplish possible an isenthalpic formation transformation of hydrates dissipating and ice. Thus, in the Gavettethe plantpressure the energy gas must contained be suitably in the fluid. heated The expansion before its process expansion would (aresult process in an commonlyexcessive known as Gas Preheatingreduction of Process). the temperature With theof the actual expanded throttling gas affecting valves, the safety the gas of the temperature plant due to the drops risk downof about freezing of the moving parts and to the possible formation of hydrates and ice. Thus, in the Gavette 7 ˝C–10 ˝C, so needing a specific preheating heat of about 15 kJ/Sm3 . plant the gas must be suitably heated before its expansion (a processNG commonly known as Gas EnergyPreheating requirements, Process). With both the electrical actual throttling and thermal,valves, the ofgas thetemperature other activities drops down of about the 7 district °C– can be 3 observed,10 in °C, order so needing to study a specific the trends preheating through heat of dataabout coming15 kJ/Sm fromNG . the various energy meters currently in use. FigureEnergy3 shows requirements, the trend both of theelectrical total and daily thermal, district’s of the thermalother activities demand of the fordistrict building’s can be heating and Domesticobserved, Hot in Water order to (DHW) study the purposes, trends through varying data coming with the from average the various environmental energy meters currently temperature and the heatingin use. season. Figure Figure 3 shows4 proposesthe trend of insteadthe total daily the values district’s of thermal the electric demand district’s for building’s demand heating which, on and Domestic Hot Water (DHW) purposes, varying with the average environmental temperature and the contrary,the heating is constant season. throughout Figure 4 proposes the year.insteadIn the order values toof pursuethe electric a district’s better time demand granularity, which, on a further model tothe simulate contrary, the is constant demand throughout time history the year. could In order have to pursue been a adopted better time [27 granularity,], but this a further refinement was consideredmodel beyond to simulate the current the demand targets. time history could have been adopted [27], but this refinement was Actually,considered the thermalbeyond the demand current targets. is supplied by two liquid water natural gas boilers while the Actually, the thermal demand is supplied by two liquid water natural gas boilers while the national gridnational provides grid provides electricity. electricity.

30 25

3 20 15

Million Sm 10

Natural gas demand 5 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 2. Monthly volumes of natural gas (NG) processed in the expansion plant. Figure 2. Monthly volumes of natural gas (NG) processed in the expansion plant.

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Sustainability 2015, 7, page–page 18 1816 1614 1412 1210 108 86 64 42 DHN heat demand [MWh/day] demand DHN heat 20 DHN heat demand [MWh/day] demand DHN heat 0 5 10 15 20 25 30 0 0 5 10Average daily 15 temperature °C 20 25 30 Average daily temperature °C Heating ON regression curve Heating OFF regression curve Heating ON regressionmeasured datacurve Heating OFF regressionmeasured datacurve

Heating ON measured data Heating OFF measured data FigureFigure 3. 3.Variation Variation of of the theaggregated aggregated daily building daily ther buildingmal demand thermal with the average demand daily withtemperature. the average daily temperature.Figure 3. Variation of the aggregated daily building thermal demand with the average daily temperature. 140

140120

120100

10080

8060 MWh/month

Electrical demand 6040 MWh/month Electrical demand 4020

200 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4. Monthly aggregated electric energy demand of the district.

FigureFigure 4. Monthly 4. Monthly aggregated aggregated electricelectric energy energy demand demand of the of district. the district. 3.2. The Demonstrator Design

3.2. The Demonstrator3.2. TheAs Demonstratorabovementioned, Design Design CELSIUS provides the realization of plants to demonstrate and promote innovative technologies on renewable and waste energies. As abovementioned, CELSIUS provides the realization of plants to demonstrate and promote The Genoa’s demonstrator will be built in the Gavette district and will promote improvements As abovementioned,innovative technologies CELSIUS on renewable provides and waste the energies. realization of plants to demonstrate and promote along the entire energy chain: from production through distribution to the final users (Figure 5). innovative technologiesThe Genoa’s demonstrator on renewable will and be built waste in the energies. Gavette district and will promote improvements The strength of the project is the recovery of energy from the pressure drop required for the along the entire energy chain: from production through distribution to the final users (Figure 5). The Genoa’sdistribution demonstrator of natural gas [28–30]. will be In built parallel in to the the Gavette current expansion district valves and will a turboexpander promote improvements will The strength of the project is the recovery of energy from the pressure drop required for the be introduced, with the aim of converting part of the mechanical energy of the gas into electricity that along thedistribution entire energy of natural chain: gas from [28–30]. production In parallel to through the current distribution expansion valves to the a turboexpander final users (Figure will 5). can be consumed by users of the district or sell in the national electric market (Figure 6). The strengthbe introduced, of thewith projectthe aim of is converting the recovery part of the of energymechanical from energy the of pressurethe gas into dropelectricity required that for the The turbo expansion of natural gas is still rarely used in distribution systems at town levels, due distributioncan ofbe naturalconsumed gas by users [28– 30of the]. Indistrict parallel or sell to in thethe national current electric expansion market (Figure valves 6). a turboexpander will to the high costs of the machines. The turbo expansion of natural gas is still rarely used in distribution systems at town levels, due be introduced,The with technological the aim oflevel converting of this radial part turbomachinery of the mechanical is very energyhigh. Though of the the gas temperatures into electricity that to the high costs of the machines. involved are substantially lower than a gas turbine, many other issues are due to the type of working can be consumedThe technological by users of level the districtof this radial or sell turbomachinery in the national is very electric high. Though market the (Figure temperatures6). The turboinvolved expansion are substantially of natural lower than gas a is gas still turbine, rarely many used other in distributionissues are due to systems the type of at working town levels, due to the high costs of the machines. 7 The technological level of this radial turbomachinery7 is very high. Though the temperatures involved are substantially lower than a gas turbine, many other issues are due to the type of working fluid, the natural gas. In particular, any leakage of gas must be recirculated in the distribution circuit to avoid the creation of an explosive environment. It is worth noting that that the turboexpander must ensure the proper pressure distribution of the downstream line. The operating characteristics of the machine must therefore be adaptable to different load conditions, such as gas flow rate and inlet pressure, in order to maintain the outlet pressure in a close range of variation. The machine then adjusts the downstream pressure, keeping

16709 Sustainability 2015, 7, 16703–16719 high conversion yields, through a stator angle control and the rotation speed. An expansion valve is often placed upstreamSustainability 2015 of, 7 the, page–page turbomachine, as an additional tool to control the gas flow evolving and to reduce thefluid, pressure the natural fluctuations. gas. In particular, any Due leakage to theof gas high must be nominal recirculated in speed the distribution of rotation, circuit it is possible to support the axis ofto the avoid machine the creation throughof an explosive magnetic environment. bearings, as in the case for the Genoa demonstrator. It is worth noting that that the turboexpander must ensure the proper pressure distribution of The correct sizingthe downstream of the line. turboexpander The operating characteristics is crucial of the machine for maximizing must therefore be the adaptable energy to recovered and minimizing the paybackdifferent load period. conditions, In such this as kind gas flow of rate delicate and inletprocedure, pressure, in order a goodto maintain knowledge the outlet of the seasonal pressure in a close range of variation. The machine then adjusts the downstream pressure, keeping and most importantlyhigh conversion the hourly yields, trendsthrough a ofstator the angle gas control flow and demand the rotation are speed. required, An expansion in valve addition is to the steady characteristics of theoften plantplaced upstream such asof the operating turbomachine, pressures, as an additional in tool order to control to the maximize gas flow evolving the and flow expanded by to reduce the pressure fluctuations. Due to the high nominal speed of rotation, it is possible to support the turboexpansionthe lineaxis ofinstead the machine of through the conventionalmagnetic bearings, as valves in the case line. for the Genoa demonstrator. The correct sizing of the turboexpander is crucial for maximizing the energy recovered and The characteristicminimizing curve the payback of period. the In selected this kind of de turboexpanderlicate procedure, a goodis knowledge standard of the seasonal for that category of machineries and presentsand most importantly the following the hourly trends features: of the gas flow demand are required, in addition to the steady characteristics of the plant such as operating pressures, in order to maximize the flow expanded by ‚ if the gas demandthe turboexpansion exceeds line the instead maximum of the conventional value valves (about line. 120% of the design mass flow), it means The characteristic curve of the selected turboexpander is standard for that category of that the conventionalmachineries and valves presents line the following processes features: the surplus; ‚ if the gas demand• if the isgas lower demand exceeds than the maximum minimum value (abo theut 120% machine of the design stops, mass flow), due it to means the low conversion that the conventional valves line processes the surplus; performance,• andif the the gas whole demand flowis lower leads than the through minimum the the machine valves stops, line. due to the low conversion performance, and the whole flow leads through the valves line.

Figure 5. Demonstrator’s energy flows. Figure 5. Demonstrator’s energy flows. Sustainability 2015, 7, page–page

8

Figure 6. Sketch of the natural gas expansion plant: (1) Throttling valves line gas preheater; Figure 6. Sketch(2) Throttling of the valve; natural (3) Turboexpander gas expansion line preheater; plant: (4) Flow (1 regulation) Throttling valve; (5) valves Turboexpander. line gas preheater; (2) Throttling valve; (3) Turboexpander line preheater; (4) Flow regulation valve; (5) Turboexpander. The turboexpander is capable of delivering a nominal power of 550 kWe. The design working flow rate is 22,500 Sm3/h, a value that is slightly lower than the average hourly flow rate of the system, corresponding to a minimum flow rate of about 3500 Sm3/h ensuring that the machine operates a long time even when demand is low. 16710 Proper sizing of the unit must ensure an effective use of the machine throughout the year; a check can be carried out by verifying some natural gas line operative parameters: • the turboexpander operation ratio, defined as the ratio between the monthly NG mass elaborated and the processable one in nominal conditions, which must be as close as possible to the unit in order to maximize the hours of steady state operation of the machine; • the throttling valves versus turboexpander flow rate ratio, defined as the ratio between the monthly mass of gas processed by the conventional gas line and the one processed by the turbomachine, which must be as low as possible, ideally zero, to minimize the unrecovered energy. The monthly trends of the aforementioned parameters are displayed in Figure 7. Note that the coefficient of use of the turboexpander assumes values close to unity during high demand periods, when the turboexpander is able to process approximately from a half to two thirds of the entire gas volumes. On the contrary, the minimum values correspond to low demand periods, when the turboexpander operates approximately from 30%–50% of its potential whilst processing almost all the required gas. Despite the implementation of the turboexpander, preheating of the gas is necessary whenever an expansion, through dissipative valves, occurs. In fact, the turbo expansion process can cause critical cooling of the gas, thus the fluid must be heated up to about 75 °C by introducing a heat demand, per kg of gas processed, approximately six times higher than the conventional isenthalpic expansion. In order to improve the effectiveness of the whole process, a CHP unit is introduced, connected to the thermal network in series to the current boilers [31,32]. The thermal production plant must supply heat to different users; below the diverse heat demands and temperature requirements are reported:

• gas preheating for the turboexpander line, about 600 kWth at 85 °C; • gas preheating for the throttling valves line, about 200 kWth at a minimum temperature of 40 °C; 9 Sustainability 2015, 7, 16703–16719

The turboexpander is capable of delivering a nominal power of 550 kWe. The design working flow rate is 22,500 Sm3/h, a value that is slightly lower than the average hourly flow rate of the system, corresponding to a minimum flow rate of about 3500 Sm3/h ensuring that the machine operates a long time even when demand is low. Proper sizing of the unit must ensure an effective use of the machine throughout the year; a check can be carried out by verifying some natural gas line operative parameters:

‚ the turboexpander operation ratio, defined as the ratio between the monthly NG mass elaborated and the processable one in nominal conditions, which must be as close as possible to the unit in order to maximize the hours of steady state operation of the machine; ‚ the throttling valves versus turboexpander flow rate ratio, defined as the ratio between the monthly mass of gas processed by the conventional gas line and the one processed by the turbomachine, which must be as low as possible, ideally zero, to minimize the unrecovered energy.

The monthly trends of the aforementioned parameters are displayed in Figure7. Note that the coefficient of use of the turboexpander assumes values close to unity during high demand periods, when the turboexpander is able to process approximately from a half to two thirds of the entire gas volumes. On the contrary, the minimum values correspond to low demand periods, when the turboexpander operates approximately from 30%–50% of its potential whilst processing almost all the required gas. Despite the implementation of the turboexpander, preheating of the gas is necessary whenever an expansion, through dissipative valves, occurs. In fact, the turbo expansion process can cause critical cooling of the gas, thus the fluid must be heated up to about 75 ˝C by introducing a heat demand, per kg of gas processed, approximately six times higher than the conventional isenthalpic expansion. In order to improve the effectiveness of the whole process, a CHP unit is introduced, connected to the thermal network in series to the current boilers [31,32]. The thermal production plant must supply heat to different users; below the diverse heat demands and temperature requirements are reported:

˝ ‚ gas preheating for the turboexpander line, about 600 kWth at 85 C; ˝ ‚ gas preheating for the throttling valves line, about 200 kWth at a minimum temperature of 40 C; ‚ building’s district heating substations for the Fire station, offices and workshops; the thermal plant can, in the demonstrator configuration, supply only 900 kWth, where the demand peak is 1500 kWth, to the district heating network (DHN) due to the low diameter of the piping system. The unit is sized to meet, in the steady state, the heat demand of the turboexpander line; in transient conditions the surplus heat produced by the CHP can be used by other thermal users. The CHP chosen is an internal combustion turbocharged engine fueled with natural gas capable of delivering, in nominal conditions, 550 kWe and to recover about 630 kWth from the engine coolant, the intercooling process and the exhaust. In analogy with the design of the turboexpander, the correct sizing of the unit can be verified by analyzing some typical operating parameters of the heat production system as:

‚ the CHP operation ratio, defined as the ratio of energy actually produced and that ideally producible in nominal conditions; ‚ the boilers versus CHP ratio, defined as the ratio of the heat produced by boilers compared to the CHP one.

The monthly trend of these two parameters is shown in Figure8; it is worth noting that the CHP coefficient of use reaches high values, greater than 80%, in the ranges where heat demand is high. In this condition, the machine is able to supply about half of the total load. The CHP coefficient of use decreases down to values in a range from 40% to 55% during low load conditions, although responding to all the heat demand without any boiler’s activation.

16711 SustainabilitySustainability 2015 2015, ,7 7, ,page–page page–page • • building’sbuilding’s district district heating heating substations substations for for the the Fire Fire station, station, offices offices and and workshops; workshops; the the thermal thermal plant can, in the demonstrator configuration, supply only 900 kWth, where the demand peak is plant can, in the demonstrator configuration, supply only 900 kWth, where the demand peak is 1500 kWth, to the district heating network (DHN) due to the low diameter of the piping system. 1500 kWth, to the district heating network (DHN) due to the low diameter of the piping system. TheThe unitunit isis sizedsized toto meet,meet, inin ththee steadysteady state,state, thethe heatheat demanddemand ofof thethe turboexpanderturboexpander line;line; inin transienttransient conditions conditions the the surplus surplus heat heat produced produced by by the the CHP CHP can can be be used used by by other other thermal thermal users. users. TheThe CHP CHP chosen chosen is is an an internal internal combustion combustion turboc turbochargedharged engine engine fueled fueled with with natural natural gas gas capable capable of delivering, in nominal conditions, 550 kWe and to recover about 630 kWth from the engine coolant, of delivering, in nominal conditions, 550 kWe and to recover about 630 kWth from the engine coolant, thethe intercooling intercooling process process and and the the exhaust. exhaust. InIn analogy analogy with with the the design design of of the the turboexpander, turboexpander, th thee correct correct sizing sizing of of the the unit unit can can be be verified verified by by analyzinganalyzing some some typical typical operating operating parameters parameters of of the the heat heat production production system system as: as: • • thethe CHPCHP operationoperation ratioratio, , defineddefined asas thethe ratioratio ofof energyenergy actuallyactually producedproduced andand thatthat ideallyideally producibleproducible in in nominal nominal conditions; conditions; • • thethe boilers boilers versus versus CHP CHP ratio ratio, ,defined defined as as the the ratio ratio of of the the heat heat produced produced by by boilers boilers compared compared to to the the CHPCHP one. one. TheThe monthly monthly trend trend of of these these two two parameters parameters is is shown shown in in Figure Figure 8; 8; it it is is worth worth noting noting that that the the CHP CHP coefficientcoefficient of of use use reaches reaches high high valu values,es, greater greater than than 80%, 80%, in in the the ranges ranges where where heat heat demand demand is is high. high. In In thisthis condition, condition, the the machine machine is is able able to to supply supply about about half half of of the the total total load. load. SustainabilityTheThe CHP CHP2015 ,coefficient 7coefficient, 16703–16719 of of use use decreases decreases down down to to values values in in a a range range from from 40%–55% 40%–55% during during low low load load conditions,conditions, although although responding responding to to all all the the heat heat demand demand without without any any boiler’s boiler’s activation. activation.

FigureFigure 7. 7. Natural Natural gas gas lines lines operational operational parameters. parameters.

Figure 8. Thermal production side operational parameters. FigureFigure 8. 8.Thermal Thermal production production side side operational operational parameters. parameters. 1010 4. Energy Analysis of the Demonstrator In order to assess the behavior and the performances of the demonstrator, a physical- mathematical model was formulated to simulate the dynamic operations of the entire district, based on Matlab-Simulink software [33]. The model is, when possible, simply implemented starting from the standard elements proposed by Matlab-Simulink for each component, associated to the appropriate mass and energy balances. In other cases, (for example to simulate the behavior of complex machines such as the turboexpander or the CHP) to obtain more accurate and reliable models, the measured characteristic curves of the components are implemented. All components are then connected and the control system is added for the regulation parameters as, for example, the power supplied by the thermal units, the NG flow to the turboexpander and the variable speed pumps of the DHN. The model requires several input parameters: some of them are directly measured data (as for the hourly demand of natural gas and the average outside temperature) while others (such as for the thermal demand of the DHN) are derived from a post-processing of the measurements (see Section 3.2). The model outputs can be temperature, pressure, mass flow, power, and energy with different sample times; for example, Figure9 shows a main model output: the NG hourly flow rate share between the two parallel expansion systems for five different characteristic days. It is observed that

16712 Sustainability 2015, 7, page–page

4. Energy Analysis of the Demonstrator In order to assess the behavior and the performances of the demonstrator, a physical-mathematical model was formulated to simulate the dynamic operations of the entire district, based on Matlab- Simulink software [33]. The model is, when possible, simply implemented starting from the standard elements proposed by Matlab-Simulink for each component, associated to the appropriate mass and energy balances. In other cases, (for example to simulate the behavior of complex machines such as the turboexpander or the CHP) to obtain more accurate and reliable models, the measured characteristic curves of the components are implemented. All components are then connected and the control system is added for the regulation parameters as, for example, the power supplied by the thermal units, the NG flow to the turboexpander and the variable speed pumps of the DHN. The model requires several input parameters: some of them are directly measured data (as for the hourly demand of natural gas and the average outside temperature) while others (such as for the thermal Sustainabilitydemand2015 of ,the7, 16703–16719 DHN) are derived from a post-processing of the measurements (see Section 3.2). The model outputs can be temperature, pressure, mass flow, power, and energy with different sample times; for example, Figure 9 shows a main model output: the NG hourly flow rate share in thebetween days characterized the two parallel by expansion a low gas systems request for the five laminating different characteristic system is activated days. It is forobserved a limited that time, duringin the peaks, days andcharacterized when the by flow a low rate gas isrequest lower the than laminating the limits system of the is activated machine. for Using a limited daily time, average values,during it is peaks, not possible and when to achievethe flow thisrate behavioris lower than which the wouldlimits of involve the machine. in less Using precise daily results. average values,To discuss it is not the possible energy to aspects achieve of this the behavior system which a campaign would ofinvolve simulations in less precise was undertaken results. in order to characterizeTo discuss an the entire energy year aspects of operation; of the system the modela campaign results of simulations are first processed was undertaken to obtain in order monthly to characterize an entire year of operation; the model results are first processed to obtain monthly behavior, and second to obtain summary annual reports. behavior, and second to obtain summary annual reports. A descriptionA description of theof the thermodynamic thermodynamic andand numerical model model of ofthe the CHP CHP and and of the of turboexpander the turboexpander can becan found be found in [in34 [34,35].,35].

Figure 9. NG flow rate sharing between turboexpander and valves for different characteristic days. Figure 9. NG flow rate sharing between turboexpander and valves for different characteristic days. 4.1. Monthly Analysis 4.1. Monthly Analysis This paragraph describes the energy aspects of the system observing the monthly trends of the mainThis plant paragraph parameters. describes the energy aspects of the system observing the monthly trends of the main plant parameters. The trend of the gas distribution between the turboexpander system is shown in Figure 10, where the gas pressure energy is recovered by the machine11 and by the throttling valves, where energy is wasted. the high seasonality of gas demand is clearly shown, with high gas demands in the heating season, when the machine collects the waste energy from about half of the monthly gas demand. During the warmer months, when the volumes of gas required are lower, the machine is able to elaborate almost all the monthly gas demand inducing a higher performance in the energy recovery. The trends of the thermal demand of the entire plant are represented in Figure 11, formed by the heat consumption of the DHN and by the two gas preheating devices. The DHN thermal demand has a strong seasonal trend and is definitely linked to the weather conditions. It is worth noting that, although the volume of gas drawn by the two expansion systems remains the same, they require different kinds of energies. The thermal preheating demand of the turbo-expander system is always the greater thermal load, assessing between 50% in winter and 80% in summer of the total thermal load. An overview of the thermal production system is shown in Figure 12. The CHP unit and the boilers are connected in series and the latter have the purpose of covering the peaks when the heat demand exceeds the CHP capability. It is worth noting that, thanks to the high thermal loads required by the gas preheating of the turboexpander line present also in the hottest periods, the CHP unit operates with good coefficients of use throughout the whole year.

16713 SustainabilitySustainability 20152015,, 77,, page–pagepage–page

TheThe trendtrend ofof thethe gasgas distributiondistribution betweenbetween thethe turboexpanderturboexpander systemsystem isis shownshown inin FigureFigure 10,10, wherewhere thethe gasgas pressurepressure energyenergy isis recoveredrecovered byby thethe machinemachine andand byby thethe throttlingthrottling valves,valves, wherewhere energyenergy isis wasted.wasted. thethe highhigh seasonalityseasonality ofof gasgas demanddemand isis clclearlyearly shown,shown, withwith highhigh gasgas demandsdemands inin thethe heatingheating season,season, whenwhen thethe machinemachine collectscollects thethe wastewaste enerenergygy fromfrom aboutabout halfhalf ofof thethe monthlymonthly gasgas demand.demand. DuringDuring thethe warmerwarmer months,months, whenwhen thethe volumesvolumes ofof gagass requiredrequired areare lower,lower, thethe machinemachine isis ableable toto elaborateelaborate almostalmost allall thethe monthlymonthly gasgas demanddemand induciinducingng aa higherhigher performanceperformance inin thethe energyenergy recovery.recovery. TheThe trendstrends ofof thethe thermalthermal demanddemand ofof thethe entireentire plplantant areare representedrepresented inin FigureFigure 11,11, formedformed byby thethe heatheat consumptionconsumption ofof thethe DHNDHN andand byby thethe twotwo gagass preheatingpreheating devices.devices. TheThe DHNDHN thermalthermal demanddemand hashas aa strongstrong seasonalseasonal trendtrend andand isis definitelydefinitely linkedlinked toto thethe weatherweather conditions.conditions. ItIt isis worthworth notingnoting that,that, althoughalthough thethe volumevolume ofof gasgas drawndrawn byby thethe twotwo expansionexpansion systemssystems remainsremains thethe same,same, theythey requirerequire differentdifferent kindskinds ofof energienergies.es. TheThe thermalthermal preheatingpreheating demanddemand ofof thethe turbo-turbo- expanderexpander systemsystem isis alwaysalways thethe greatergreater thermalthermal loload,ad, assessingassessing betweenbetween 50%50% inin winterwinter andand 80%80% inin summersummer ofof thethe totaltotal thermalthermal load.load. AnAn overviewoverview ofof thethe thermalthermal productionproduction systemsystem isis shownshown inin FigureFigure 12.12. SustainabilityTheThe2015 CHPCHP, 7, 16703–16719unitunit andand thethe boilersboilers areare connectedconnected inin seriesseries andand thethe latterlatter havehave thethe purposepurpose ofof coveringcovering thethe peakspeaks whenwhen thethe heatheat demanddemand exceedsexceeds thethe CHPCHP capabilitcapability.y. ItIt isis worthworth notingnoting that,that, thanksthanks toto thethe highhigh thermalthermal loadsloads requiredrequired byby thethe gasgas preheatingpreheating ofof ththee turboexpanderturboexpander lineline presentpresent alsoalso inin thethe hottesthottest The chartperiods,periods, in thethe Figure CHPCHP unitunit 13 operatesshowsoperates thewithwith trends goodgood cocoefficients ofefficients electricity ofof useuse throughout production.throughout thethe whole Thewhole electric year.year. energy produced follows a seasonalTheThe chartchart trend inin FigureFigure as 1313 well, showsshows and thethe trends moretrends ofof than elecelectricitytricity half production.production. of it is produced TheThe electricelectric from energyenergy the producedproduced turboexpander followsfollows aa seasonalseasonal trendtrend asas well,well, andand moremore thanthan halfhalf ofof itit isis producedproduced fromfrom thethe turboexpanderturboexpander throughthrough through aaa recovery recovery process,process, process, whilewhile while thethe remainingremaining the remaining partpart isis stilstilll part producedproduced is still byby thethe produced CHPCHP withwith by highhigh the fuelfuel CHP conversionconversion with high fuel conversionperformances.performances. performances.

3030 2525 3 3 2020 1515

share 10

share 10

Million Sm 5 Million Sm 5 00 Natural gas mass flow mass Natural gas Natural gas mass flow mass Natural gas JanJan Feb Feb Mar Mar Apr Apr May May Jun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov Nov Dec Dec TurboexpanderTurboexpander ThrottlingThrottling valvesvalves

FigureFigureFigure 10. 10.10.Natural NaturalNatural GasGas Gas flowflow flow share.share. share.

500500 400400 300300 200200 MWh/month Heat demand MWh/month Heat demand 100100 00 JanJan Feb Feb Mar Mar Apr Apr May May Jun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov Nov Dec Dec TurboexpanderTurboexpander preheatingpreheating DHNDHN ThrottlingThrottling valvesvalves preheatingpreheating

FigureFigure 11.11. ThermalThermal demanddemand share.share. SustainabilitySustainability 2015 2015, ,7 7, ,page–page page–page Figure 11. Thermal demand share.

600600 500500 400 400 1212 300300 200200 MWh/month MWh/month Heat production Heat production 100100 00 JanJan Feb Feb Mar Mar Apr Apr May May Jun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov Nov Dec Dec CHPCHP BoilersBoilers

FigureFigureFigure 12. 12. 12.Thermal Thermal Thermal production production production share. share. share.

500500

400400

300300

200200 MWh/month MWh/month 100100 Electrical production Electrical production 00 JanJan Feb Feb Mar Mar Apr Apr May May Jun Jun Jul Jul Aug Aug Sep Sep Oct Oct Nov Nov Dec Dec CHPCHP TurboexpanderTurboexpander

FigureFigureFigure 13. 13. 13.Electrical Electrical Electrical production production production share. share. share.

4.2.4.2. Annual Annual Analysis Analysis ThisThis paragraph paragraph describes describes the the energy energy balance balance16714 of of the the system system on on a a yearly yearly basis. basis. TableTable 2 2 shows shows the the energy, energy, in in GWh GWh per per year, year, consumed consumed and and produced, produced, in in the the main main processes processes of of thethe plant.plant. TableTable 33 showsshows thethe percentagepercentage ofof thethe ratioratio ofof energyenergy utilizationutilization divideddivided byby energyenergy productionproduction both both for for the the thermal thermal and and for for the the electrical electrical part. part.

TableTable 2. 2. Yearly Yearly energy energy consumption consumption and and production. production.

EnergyEnergy Flow Flow Nomenclature Nomenclature GWh/year GWh/year CHPCHP Fuel Fuel consumption consumption F Fchpchp 6.66.6 CHPCHP heat heat production production H Hchpchp 3.53.5 CHPCHP electricity electricity production production E Echpchp 2.32.3 BoilersBoilers fuel fuel consumption consumption F Fboilerboiler 1.91.9 BoilersBoilers heat heat production production H Hboilerboiler 1.81.8 TurboexpanderTurboexpander heat heat consumption consumption H Hturboturbo 3.13.1 TurboexpanderTurboexpander electricity electricity production production E Eturboturbo 2.92.9 ThrottlingThrottling valves valves he heatat consumption consumption H Hvalvesvalves 0.40.4 DHNDHN heat heat consumption consumption H Hdhndhn 1.81.8 DistrictDistrict electricity electricity consumption consumption E Eselfself 3.73.7

1313 Sustainability 2015, 7, 16703–16719

4.2. Annual Analysis This paragraph describes the energy balance of the system on a yearly basis. Table2 shows the energy, in GWh per year, consumed and produced, in the main processes of the plant. Table3 shows the percentage of the ratio of energy utilization divided by energy production both for the thermal and for the electrical part.

Table 2. Yearly energy consumption and production.

Energy Flow Nomenclature GWh/year

CHP Fuel consumption Fchp 6.6 CHP heat production Hchp 3.5 CHP electricity production Echp 2.3 Boilers fuel consumption Fboiler 1.9 Boilers heat production Hboiler 1.8 Turboexpander heat consumption Hturbo 3.1 Turboexpander electricity production Eturbo 2.9 Throttling valves heat consumption Hvalves 0.4 DHN heat consumption Hdhn 1.8 District electricity consumption Eself 3.7

SustainabilitySustainability 2015, 7, page–page 2015, 7 , page–page SustainabilitySustainability 2015, 7Table, page–page 2015 3., 7 Yearly, page–page thermal/electrical energies sources/sinks.

Table 3. YearlyTable thermal/electrical 3. Yearly thermal/electrical energies sources/sinks. energies sources/sinks. Table 3. YearlyTable thermal/electrical 3. Yearly thermal/electrical energies sources/sinks. energies sources/sinks. SourcesSourcesSources Sinks Sinks Sinks SourcesSources Sinks Sinks

Thermal Thermal ThermalThermal Thermal

Electrical Electrical ElectricalElectrical Electrical

The energyThe consumption energy consumption of the whole of the district whole is aboutdistrict 1.8 is GWh/yearabout 1.8 GWh/year of heat and of 1.5heat GWh/year and 1.5 GWh/year The energyThe consumption energy consumption of the whole of thedistrict whole is aboutdistrict 1.8 is GWh/yearabout 1.8 GWh/year of heat and of 1.5 heat GWh/year and 1.5 GWh/year of electricity.of electricity. The turboexpander The turboexpander is able to producis able eto about produc 2.9e GWh/yearabout 2.9 GWh/year of electricity of electricitywhilst it need whilst it need of electricity.of electricity. The turboexpander The turboexpander is able to producis able eto about produc 2.9e GWh/yearabout 2.9 GWh/year of electricity of electricitywhilst it need whilst it need The energy3.1 GWh/year consumption3.1 GWh/year of heat, which ofof heat, the roughly wholewhich corresponds roughly district corresponds to is 58% about of tothe 58% 1.8 total of GWh/year heat the totaldemand heat of ofdemand the heat district. of and the It district. 1.5 GWh/year It 3.1 GWh/year3.1 GWh/year of heat, which of heat, roughly which corresponds roughly corresponds to 58% of theto 58% total of heat the demandtotal heat of demand the district. of the It district. It is worth notingis worth that, noting in the that, turboexpander, in the turboexpander, the efficiency the efficiencyof conversion of conversion of heat into of electricityheat into electricityis more is more of electricity.is The worth turboexpander notingis worth that, noting in the that, turboexpander, is in able the turboexpander, to produce the efficiency the about ofefficiency conversion 2.9 of GWh/year conversion of heat into of electricity heat of electricity into iselectricity more whilst is more it need than 0.90. than 0.90. 3.1 GWh/yearthan of 0.90. heat, than which 0.90. roughly corresponds to 58% of the total heat demand of the district. It is The CHP Thesupplies CHP 62%supplies of the 62% yearly of the heat yearly demand heat anddemand it represents and it represents 46% of the 46% electrical of the electrical The CHP Thesupplies CHP 62%supplies of the 62% yearly of the heat yearly demand heat anddemand it represents and it represents 46% of the 46% electrical of the electrical worth notingproduction, that, inproduction, thewith a turboexpander, total with fuel a prtotalocess fuel efficiency pr theocess efficiency efficiencyof about 0.86. of ofabout conversion 0.86. of heat into electricity is more production,production, with a total with fuel a pr totalocess fuel efficiency process ofefficiency about 0.86. of about 0.86. The two electricityThe two electricityproduction production units, the units,CHP anthed CHPthe turboexpander, and the turboexpander, are largely are able largely to meet able to meet than 0.90. The two electricityThe two electricityproduction production units, the CHPunits, an thed theCHP turboexpander, and the turboexpander, are largely are able largely to meet able to meet the district’sthe demands district’s asdemands they generate as they a generate surplus ofa surplus about 3.7 of GWh/year.about 3.7 GWh/year. the district’sthe demands district’s asdemands they generate as they a generate surplus ofa surplusabout 3.7 of GWh/year.about 3.7 GWh/year. The CHP suppliesAbout 38%About of 62% the 38% heat of of required the heat yearly is required provided heat is byprovided auxiliary demand by boilers: auxiliary and it canboilers: it be represents noticed it can be that noticed the 46% boilers’ that ofthe boilers’ the electrical About 38%About of the 38% heat of required the heat is required provided is byprovided auxiliary by boilers: auxiliary it can boilers: be noticed it can be that noticed the boilers’ that the boilers’ share wouldshare be wouldreduced be if reduced a larger ifCHP a larger unit CHPhad been unit installed.had been Thisinstalled. would This result would in increasing result in increasing the the production, withshare awould totalshare befuel reducedwould process be if reduced a larger efficiency ifCHP a larger unit hadCHP of been about unit installed.had 0.86. been Thisinstalled. would This result would in increasing result in increasing the the productionproduction electric energy electric and, energy thus, and, in decreasi thus, inng decreasi the shareng theof self-consumption share of self-consumption (currently (currently about about The twoproduction electricityproduction electric production energy electric and, energy units,thus, and,in thedecreasi thus, CHP inng decreasi the and shareng the ofthe self-consumption turboexpander, share of self-consumption (currently are largely (currentlyabout able about to meet 30%). This30%). appears This to appears be a profitable to be a profitableeffect because, effect at because, present, at in present, Italy, the in electricity,Italy, the electricity, that is sold that to is sold to 30%). This30%). appears This to appears be a profitable to be a profitableeffect because, effect at because, present, at in present, Italy, the in electricity, Italy, the electricity, that is sold that to is sold to the district’s demandsthe nationalthe grid, asnationalthey is poorly grid, generate remuneratedis poorly aremunerated surplus and the profit ofand about sthe (per profit unit 3.7s of (per GWh/year.fuel unit burned) of fuel tend burned) to decrease tend to by decrease by the nationalthe grid, national is poorly grid, remunerated is poorly remunerated and the profit ands the (per profit units of (per fuel unit burned) of fuel tend burned) to decrease tend to by decrease by increasingincreasing the share ofthe energy share ofsold. energy sold. About 38%increasing of theincreasing the heat share required ofthe energy share sold.of is energy provided sold. by auxiliary boilers: it can be noticed that the boilers’ Other importantOther important information information deduced from deduced the annual from the energy annual analysis energy are analysis the fuel are savings. the fuel The savings. The share would be reducedOther importantOther if a important largerinformation CHP information deduced unit from deduced had the been annual from installed.the energy annual analysis energy This are analysis wouldthe fuel are savings. resultthe fuel The insavings. increasing The the key pointkey which point has which been hasconsidered been considered to make toa powerfulmake a powerfulcomparison comparison between betweendifferent different key pointkey which point has which been hasconsidered been considered to make toa powerfulmake a powerfulcomparison comparison between differentbetween different production electricconfigurations energyconfigurations of plant and, is of the thus,plant amount is in the decreasingof amount fuel saved, of fuel as the saved,suggested share as suggested by of rules self-consumption forby therules high-efficiency for the high-efficiency (currently about configurationsconfigurations of plant is of the plant amount is the of amount fuel saved, of fuel as saved,suggested as suggestedby rules for by therules high-efficiency for the high-efficiency cogenerationcogeneration [36]. The cogeneration[36]. The cogeneration system is systemcompared is co withmpared two equivalentwith two equivalent conventional conventional systems systems 30%). This appearscogeneration tocogeneration be[36]. a The profitable cogeneration [36]. The cogeneration effect system because, is cosystemmpared is at cowith present,mpared two equivalentwith in two Italy, equivalent conventional the electricity, conventional systems thatsystems is sold to which producewhich the produce same amount the same of amount energy ofand energy electricity and electricityand which and have which the efficiency have the efficiencyof the whole of the whole which producewhich the produce same amount the same of amount energy andof energy electricity and electricityand which and have which the efficiency have the ofefficiency the whole of the whole the national grid,national is energy poorlynational production. energy remunerated production. The cogeneration andThe cogeneration the syst profitsem, compared syst (perem, compared unitwith two of equivalentwith fuel two burned) equivalent conventional tend conventional to decrease by national energynational production. energy production. The cogeneration The cogeneration system, compared system, comparedwith two equivalentwith two equivalent conventional conventional systems, producessystems, producesthe same theamount same of amount energy ofan energyd electricity and electricityand has the and efficiency has the efficiencyof the whole of the whole increasing thesystems, share produces ofsystems, energy theproduces same sold. amountthe same of amount energy ofan denergy electricity and electricityand has the and efficiency has the efficiencyof the whole of the whole national energynational production. energy production. national energynational production. energy production. The energyThe scheme energy of scheme the cogeneration of the cogeneration system flows system and fl ofows the and conventional of the conventional ones is shown ones inis shown in The energyThe scheme energy of schemethe cogeneration of the cogeneration system flows system and flofows the and conventional of the conventional ones is shown ones inis shown in Figures 14Figures and 15 14applied and 15 to applied the Gavette to the area. Gavette area. Figures 14Figures and 15 applied14 and 15 to appliedthe Gavette to the area. Gavette 16715 area.

14 14 14 14 Sustainability 2015, 7, 16703–16719

Other important information deduced from the annual energy analysis are the fuel savings. The key point which has been considered to make a powerful comparison between different configurations of plant is the amount of fuel saved, as suggested by rules for the high-efficiency cogeneration [36]. The cogeneration system is compared with two equivalent conventional systems which produce the same amount of energy and electricity and which have the efficiency of the whole national energy production. The cogeneration system, compared with two equivalent conventional systems, produces the same amount of energy and electricity and has the efficiency of the whole national energy production. The energy scheme of the cogeneration system flows and of the conventional ones is shown in

SustainabilityFigures 142015 and, 7, page–page15 applied to the Gavette area.

Sustainability 2015, 7, page–page

FigureFigure 14. 14. Gavette’sGavette’s power power plant. Figure 14. Gavette’s power plant.

Figure 15. Equivalent conventional plants. Figure 15. Equivalent conventional plants.

The fuel saving, Fsaving , can be calculate from Equation (1), where Fchp and Fboilers are the

The fuelestimated saving, fuel consumptionFsaving, can of bethe district’s calculate production from Equation line, Fep and (1), F wheretp are theF chpequivalentand Ffuel'sboilers are the estimatedconsumption fuel consumption of the conventional ofFigure the district’s15. electric Equivalent and production thermal conventional Plants. line, Fplants.ep and Ftp are the equivalent fuel's consumption of the conventional electric and= thermal+ − Plants.+ Fsaving Fep Ftp (Fchp Fboilers ) (1) The fuel saving, Fsaving , can be calculate from Equation (1), where Fchp and Fboilers are the The useful effect of the demonstratorFsaving “ Fep is `givenFtp by´ ptheFchp sum` ofF boilersthe electricq energy generated by the (1) estimated fuelCHP, consumption Echp , and the turboexpander,of the district’s Eturbo production, and from the line, net heat F epoutgoing and fromFtp the are system, the equivalent H out fuel's The useful(Equation effect (2)). of the demonstrator is given by the sum of the electric energy generated by consumption of the conventional electric and thermal Plants. the CHP, Echp,The and last the one turboexpander, is the heat generatedEturbo by ,the and CHP, from H chp the plus net the heat one outgoing generated fromby thethe boilers, system, Hout = + − + (Equation (2)).H boiler : the heat absorbed Fbysaving the turboexpanderFep F tppreheating(Fchp system,Fboilers Hturbo) , is not taken into account (1) because it is an internal heat exchange and it is not provided to/from outside. In fact, it does not cross The usefulthe boundaryeffect of of the the demonstratorcontrol volume. is given16716 by the sum of the electric energy generated by the =+ − = CHP, Echp , and the turboexpander,Hout EHHturbo chp, and boiler from H turbo the2.2 net GWhyear heat / outgoing from the (2)system, H out (Equation (2)). The conventional equivalent plant must have the same electrical, Eep , and thermal, H tp The lastuseful one effectsis the of heatthe cogenerative generated one by (Equations the CHP, (3) and H (4)).chp plus the one generated by the boilers, H : the heat absorbed by the turboexpander preheating system, H , is not taken into account boiler 15 turbo because it is an internal heat exchange and it is not provided to/from outside. In fact, it does not cross the boundary of the control volume. =+ − = HoutHH chp boiler H turbo 2.2 GWhyear / (2)

The conventional equivalent plant must have the same electrical, Eep , and thermal, H tp useful effects of the cogenerative one (Equations (3) and (4)).

15 Sustainability 2015, 7, 16703–16719

The last one is the heat generated by the CHP, Hchp plus the one generated by the boilers, Hboiler: the heat absorbed by the turboexpander preheating system, Hturbo, is not taken into account because it is an internal heat exchange and it is not provided to/from outside. In fact, it does not cross the boundary of the control volume.

Hout “ Hchp ` Hboiler ´ Hturbo “ 2.2GWh{year (2)

The conventional equivalent plant must have the same electrical, Eep, and thermal, Htp useful effects of the cogenerative one (Equations (3) and (4)). National law introduces average performance of generation for each type of burned fuel [37]. For the natural gas the standard electric performance, ηel,ri f , is 0.46 and the thermal, ηth,ri f , is 0.90. The equivalent standard plants fuel consumption can be than calculated by Equations (5) and (6).

Eep “ Echp ` Eturbo (3)

Htp “ Hout (4)

Eep Fep “ “ 11.3GWh{year (5) ηel,ri f

Htp Fth “ “ 2.4GWh{year (6) ηth,ri f By means of this procedure, it has been possible to estimate by Equation (7) a Primary Energy Saving index (PES) of about 38% equal to 5.3 GWh/year of fuel energy (from Equation (1)).

Fchp ` Fboiler PES “ 1 ´ (7) Fep ` Ftp

3 In terms of natural gas consumption, the upgraded plant can save more than 500,000 SmNG per year equally to more than 1000 tons of equivalent CO2 saved emission.

5. Conclusions The EU’s plan for generating low carbon energy and mitigating climate change, calls for considerable investment to enable the rapid conversion of the EU’s existing energy plants into 21st century systems, capable of guaranteeing both energy security into the future and, in doing so, creating a more sustainable world. In order to minimize GHG emissions in urban areas a key action is the massive exploitation of the unused energy potential through the effective and efficient recovery of energy losses. Therefore, the smart integration of waste energy to raise energy efficiency and carbon savings in cities is the aim of the CELSIUS Project (Combined Efficient Large Scale Integrated Urban Systems), which is the biggest winning project within the EU call "FP7 Smart Cities & Communities 2011". The CELSIUS demonstrator built in Genoa aims to recover waste energy by the natural gas mass flow rate, expanding from the main supply line to the city natural gas network through a turbine/generator system. Moreover, the turboexpander has been combined in a CHP plant that supplies a district heating network, in order to save energy and reduce CO2 emissions by means of smart systems integration. By using a numerical simulator, it was possible to analyze the behaviour of the whole energy system and to predict energy flows and figures over different time scenarios. Dynamic modeling showed that the demonstrator is able to save a considerable amount of fuel and to generate profit from the sale of electricity and heat. A total amount of produced electricity of almost 5 GWh/year will be generated, with a recovery of 2.9 GWh/year from the turboexpander. The CHP plant will produce 2.3 GWh/year of electricity and 3.5 GWh/year of heat. More than 500,000 Sm3/year of Natural Gas

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saving, corresponding to over 1000 tons of CO2/year, is estimated with a Primary Energy Saving index up to 35% if compared with conventional separated production of heat and electricity by NG. The goal of achieving full energy independence of the district and reduction of local emission of GHG, in accordance with the targets of the Covenant of Mayors and of the related Sustainable Energy Action Plan, is now definitely closer. Moreover, a powerful tool for the design and control of energy systems has been developed, with a flexible approach that can be adapted to the grid extension/evolution.

Acknowledgments: This work was developed in the frame of the collaborative project CELSIUS (Combined Efficient Large-Scale Integrated Urban Systems). This project received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement No. 314441. Author Contributions: All the authors equally contributed to this work. Conflicts of Interest: The authors declare no conflict of interest.

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