A state of art review on the district heating systems

Renewable and Sustainable Energy Reviews 96 (2018) 420–439

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A state of art review on the district heating systems ⁎

T

Abdur Rehman Mazhar, Shuli Liu , Ashish Shukla School of Energy, Construction and Environment, Faculty of Engineering, Environment and Computing, Coventry University, Coventry CV1 5FB, United Kingdom

A R T I C LE I N FO

A B S T R A C T

Keywords: District heating Distributed heat sources Low carbon heating

District heating (DH) has been widely acknowledged as the future for urban heating, as concerns of sustainability and greenhouse gas emissions are pushing, to revamp the heating sector. Although not a new technology, DH has evolved over four different generations, and is now capable of incorporating low temperature distributed renewable heat sources. The ongoing advancements and typical characteristics of a complete grid are researched upon in this paper. The research is extended to link up these characteristics with some of the most interesting DH networks in the world. The technical configurations of these DH grids along with their regulations and policies are explored upon, to understand their evolution and uniqueness. A basic overview of the economics and social aspects are presented as well. The overall trends and results of this study show that, as fossil fuel prices fluctuate and are depleting, there is a unified effort to promote DH, especially for residential heating purposes. The characteristics of modern DH are evolving to cater for distributed renewable technologies with the aim of making the entire system carbon free and sustainable.

1. Introduction Approximately 50% of the energy consumption in Europe is used for heating which corresponded to about 565 Mtoe of the final consumption, as of 2008 [1]. A huge potential exists to decarbonize this sector, as only 12% was supplied with a renewable form of energy while only 10% of the total demand in Europe was covered by district heating (DH) [1]. A DH grid has different central and individual heat generation sources, transmitting hot water via insulated pipes to be distributed to consumers [2]. In comparison to conventional heating technologies, DH has numerous advantages [3,4]. In densely populated urban centres, DH not only makes more economic sense but is considered, as the only viable option to decarbonize heating [4]. DH is a technology that has evolved considerably over the last four decades. During the 1980s most research focused on the basic characteristics of networks [5]. Combined heat and power (CHP) and cogeneration systems were introduced along with several control strategies including heat meters, valves and piping materials etc. [6]. Most of the research focused on the technological aspects of CHP stations along with the fuels used, with coal being at the forefront [7]. During this era, steam or hot water was the main carrier, with most studies focused on the metallurgical aspects of piping's and their insulations [5]. During the next decade DH integrated into many modern societies in Europe and North America [8–12]. Within this era most research focused on operational optimization to enhance the energetic along

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with exergetic efficiencies. Costs were another important criterion, hence thermodynamic, techno economical and optimization studies were the main research focus. Supply temperatures, matching of load and supply in efficient manners, refurbishment and retrofitting of transmission networks along with substations were focused upon. During the early 21st century the focal point in DH shifted to more sustainable systems with lesser carbon emissions [13–16]. At this point of time, renewable technologies had gained momentum and the concept of decentralized sustainable grids was in place. Researchers focused on renewable heating technologies especially biofuels and solar thermal systems. The pivotal point of every research publication during this time was to reduce greenhouse gas emissions, within DH networks. At the same time studies focused on reduction of space heating demands and load control strategies emerged. Over the last couple of years the focus of scientific studies has once again shifted to a more integrated and large-scale approach [17–20]. In this new era, the idea of the overall smart integration of different energy grids seems of importance for many researchers. Decentralized renewable heating technologies have evolved over the recent years, paving the way for low temperature fourth generation DH, resulting in the evolution of overall smart integrated decentralized grids. Many decentralized renewable technologies are connected to the grid, with the flow of energy and information both ways between the grid and the consumer [21]. At the same time the interlinking of heat and electricity generation technologies have made it impossible to operate and

Corresponding author. E-mail address: [email protected] (S. Liu).

https://doi.org/10.1016/j.rser.2018.08.005 Received 26 September 2017; Received in revised form 4 August 2018; Accepted 5 August 2018 1364-0321/ Crown Copyright © 2018 Published by Elsevier Ltd. All rights reserved.

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ranked according to economical, energy and environmental factors [24] using a fuzzy comprehensive evaluation method. The overall ranking of the best systems was CHP > gas boiler > water source heat pumps > coal boilers > ground source heat pumps > solar energy heat pumps > oil boilers, with respect to these three assessment criteria's. The algorithm used is based on both numerical data and several decision criteria to ensure that qualitative, quantitative, subjective and objective factors are considered.

manage one form of energy without considering the other [22]. Peak heat generation boilers, are being replaced by different forms of latent and sensible heat storage, where studies have been focusing on flattening the heat load curves by utilizing thermal storage. Finally, different network topologies have also been studied in order to integrate the different DH grids within a region, based on attitudes of different user segments [23]. Most studies in the past, focused on one aspect or domain within a DH grid without interconnecting the entire system especially considering the ongoing policies and technological developments in leading DH countries. This study focuses on an overall analysis of the integral parts of a modern DH system, along with techno-economic considerations. Also, a critical analysis of the different prospects of the grid along with grid leaders is presented, considering their policies and regulations. Some critical areas researched upon are:

2.1.1. Renewable heat sources The most preferable heat sources for a sustainable DH network, are renewable and/or waste heat sources [25], which can even be used in a low temperature heat grid [26]. Waste heat that cannot be utilized due to its low temperature, comes into use with DH, hence it is harnessed and is also considered as a third-generation renewable source. Some prominent renewable technologies used in conjunction with DH, ranked according to their Technology Readiness Level (TRL) on a scale of 1–9, as per the definition of the European Commission, are hierarchal arranged in Table 1. In summary, although solar thermal and geothermal sources are ranked TRL-9, but their potential to be used in conjunction with DH is comparatively lower compared to bioenergy, waste industrial heat and EfW sources.

• The basic characteristics of DH grids and the latest researched technologies within this domain; • An overview of the economics and social aspects of DH • The identification of market leaders in DH grids and a basic study of the evolution of these grids; and • Legislation, technological framework and policies, within these DH leaders, with linkage to the basic characteristics of grids.

2.1.2. CHP heat sources The most common cogeneration strategy, used for both the production of electricity and heat, to be used in conjunction with DH are CHP plants [7]. Without a heat network, there is no other way to transport this heat from large scale CHP plants. At the moment, it is the most common source for DH networks throughout the world as explained in Section 4, and will be an integral part of high-density urban grids of the future. CHP plants have both a higher overall efficiency and at the same time can be sourced from renewable resources [51]. If heat for domestic consumers is produced alone, the flue gases of a high temperature combustion process are utilized to produce a relatively low temperature heat source, which is exergetically not efficient. On the other hand, as in CHP plants, if the by-product of combustion, that is a lower grade heat source, produces this same heat for domestic consumers the exergetic efficiency is much higher. Hence cogeneration is favorable both in terms of energy and exergy efficiency [3]. Most of the research on CHP plants is not focused on the technology, but on improvements, hybrid combinations, fuel variations and analysis, due to it being a mature technology. Several optimization studies to enhance CHP-DH operation have been published. Modelling tools to implement control strategies of operation and management of heat from combined CHP-DH have been implemented in simulation platforms [52]. The objective of these algorithms is to thermo-economically optimize demand and supply in an integrated network [53]. Apart from Fuel-Cell CHPs, the CHP variants are all relatively mature technologies with occurrence depending on social-economic parameters. Some of the most prominent technologies within this domain are in Table 2:

2. Operation and characteristics of DH systems Many historians consider the hot water distribution system of the 14th Century in Chaudes-Aigues, a small town close to Lyon in southern France, as having the first operational DH system in the world [3]. This scheme provided hot water via geothermal sources to about 30 houses. Although several new DH systems appeared globally by the end of the nineteenth century, the basic features remain the same [2]. A DH grid can be divided into the generation side where one or more heat producing facilities, with water as the medium of heat transfer, are connected to a transmission network of pipes covering a distance to the distribution grid, where it is finally used by the endconsumers, as depicted in Fig. 1. The size of a typical DH grid may range from a housing scheme connected to a few houses or to an entire city or region, independent of the building types. 2.1. Generation An extremely vital characteristic of DH is its flexibility in the sources of heat. Many different centralized and decentralized sources can be connected for reliable operation and flexibility to a DH grid, with basic control strategies. Depending on the geographical location and requirement, the same DH grid can be used for District Cooling (DC) in the summer season, with the same heat source. Some conventional heat sources are as follows:

• CHP plants – producing both heat and electricity with different ra• • • • •

2.1.3. Thermal storage technologies for DH Thermal storages can serve as heat sinks at times of low demand and as heat sources at times of peak demand. Energy storage is expected to be an integral part of future DH due to the integration of unpredictable fluctuating renewable heat sources [61]. Thermal storages can also be used for long term usages, in underground pits or caverns (centralized) or small scale in house-hold drums (decentralized). Although not much efficient, heat can be stored in summer to be utilized in winter, in long term storages. The typical efficiency of thermal storages lie in the range of 20–60% [62], depending on the conditions In a study, by using different operational modes, the flexibility and response time of both types of storage is assessed. It is concluded that centralized storage is much more flexible and easy to manage [63]. Thermal storages are the cheapest option, even compared to electrical storage, at the moment, as it does not need any costly material conversions or infrastructure

tios or either one, including large and mCHP facilities. Can be used with a variety of fuels including renewable biomass; Waste heat – either from industrial processes, agricultural processes or even from combustion of waste itself (EfW); Geothermal heat – a renewable source found beneath the earth's surface; Solar thermal heat – both large scale and small scale setups; Heat pumps – usually large and small-scale electrically operated, when there is an excess of electricity e.g. in Scandinavia when there is an excess of wind energy; and Conventional boilers – normally used as a back-up, in events where peak load exceeds production. Can be used with a variety of fuels including renewable biomass. The seven most common heat generation technologies have been 421

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Fig. 1. Generation, transmission and distribution of typical DH.

upgrade in, order to deliver the final energy to domestic users. They will also play an important role to link electric and heat grids in the future. Sensible heat storage is the oldest form, while latent heat storage is an emerging technology. An overview of thermal storages, in conjunction with DH is summarized in Table 3:

rate variations and a ring network topology, could substantially increase the overall efficiency of a grid [73]. Ring topologies have a circular network of pipes compared to conventional topologies which have a dead-end as illustrated in Fig. 3 which increases the flexibility and reliability, The best mechanism for operation can be achieved by accurately predicting the future loads and applying demand side management techniques to balance out the load curves [74]. Several forecasting techniques have been developed with the most promising; using artificial intelligence based neural network methods. Loads with an accuracy of 95% can be predicted. However, it is argued that the versatility of fuels used for heating in urban centres, makes it quite tough to predict household heating demands, especially in case the feasibility of replacing this demand with a DH is to be studied [75]. It is important to note at this point; another advantage of DH is its benefit from ‘Diversity Factors’. The simultaneous occurrence of a reduced heating load in different buildings, in our case, as compared to the system design parameters of the building alone, can allow designers to develop an integrated system with less intensive parameters. In this way, the overall design parameters of generation are less than the sum of the individual parameters of the buildings, reducing costs. In the future, the transmission of electricity and heat would be operated and studied simultaneously. This is because of the several interlinked technologies and the fact that excess electricity from intermittent renewable technologies, would be used for heat production [76]. Excess electricity could be utilized by using electric boilers for peak shaving, in a DH network and is thought to be a common strategy in the future.

2.2. Operation Managing loads involves a comprehensive control strategy, based on the full load hours of the heat sources and their costs per unit, a priority scheme can be derived, in order to actually realize the benefits of the installed physical network [68]. The most efficient and reliable source i.e. CHP, EfW or waste industrial heat sources are normally selected during base load hours while only back-up sources i.e. large scale boilers or thermal storages are used to cover the peak load hours [69]. Usually at times of peak heat demands, the electricity and heat production are decoupled in CHP plants to enhance operational flexibility… Renewable technologies combined with thermal storages are typically used in the middle of these two extremes, along with microCHPs (mCHPs). A typical load profile curve for a multi-source DH network of a typical small city of 200,000–250,000 residents would look as Fig. 2: By using long term thermal storage, the operational time of base load equipment, the efficiency, costs and environmental benefits of the entire DH system can be enhanced substantially [61]. Several algorithms and control strategies have been formulated to define the operational strategy of heat storages and the functionality of the heat plants involved [70]. It is proved that the pumping costs and heat loss costs during operation, determine the operating costs of a DH grid [71]. Both these costs are inversely proportional to the mass flow rates and an optimum operating point must be determined based on the demand and supply [72]. It is proved that a better control strategy based only on mass flow

2.3. Transmission The transmission network supplies customers with hot water and returns the cooled water back to the source. This is done via 422

423

Waste incineration (EfW) TRL-9

Industrial waste heat TRL-9

Bioenergy TRL-9

Technology

Prospects

performance.

•

•

•

and old, heat generation mechanism. on the circumstances, the output of EfW plants can • AMostmature • Depending • be electricity, heat or biofuels, which is a competition for it to research evolving around this technology focuses on • improvements, be used in DH. infrastructure development, economic aspects and flue gas cleansing methods. Currently waste management, is quite ad-hoc in different • parts of the world, making the generation economically in many modern large scale DH networks. Japan and • Used • unviable. China are considered as market leaders in this technology (Section 4). One of the largest EfW plant generates 95 Lack of proper waste infrastructure similar bioenergy • infrastructure, as explained previously. MWe in Florida USA. • gases of combustion can be dangerous especially if waste • Flue isn’t categorized properly e.g. combustion of nonbiodegradable products. •

[33–35]

[30–32]

[27–29]

Ref.

(continued on next page)

gas emissions are lower when more biofuel is used in the transport sector as compared to the DH network, due to its limited availability. Currently large scale biofuel production for DH grids doesn’t guarantee economic benefits, but only environmental benefits especially with CHP plants. However, with proper policies this will change soon. Due to the versatile range of bio-fuels, the best practice to use them is in a bio-village with multiple energy requirements, as evident from the study in Germany. In such villages, large scale biogas and ethanol production is analyzed to show cost reductions and benefits, when used solely in DH networks. Tools are available to comprehensively plan and optimize the layout of such villages, to harness this renewable source to the maximum. Although a huge potential exists, results of studies conclude that the only hurdle is the lack of an initiative by policymakers. At least 90% of Europe's heating demand could be met by this source alone. Both high grade and low grade heat is produced requiring variable temperature cascaded DH grids, linked to both industrial and residential users. Thermal storage especially high density latent heat storage is a necessity, as explained in the previous parts of this table. 4th generation low temperature grids can harness even lowgrade industrial heat in lieu with thermal storage, to realize a huge potential of this waste. The energy output is highly dependent on the external circumstances, region and legislation. In Asia, most EfW plants generate electricity while in Europe, heat is primarily produced. Biofuels are relatively at a lower scale. Studies reveal huge potentials especially in urban centres of Europe, with proper fiscal policies to develop and manage the infrastructure. At the moment, biomass CHPs are more efficient and a cleaner option of combustion, compared to waste incineration only due to a relatively better infrastructure. Clean combustion and efficient waste management strategies are the key to this issue. Several research focusing on the clean combustion of three fuels waste, biomass and natural gas to be used in both CHP and heat only plants for DH are analyzed. The differences between them and their analogies are used to define flue gas cleansing techniques. It is likely to be an integral part of future cogeneration power plants, in Europe and Asia due to increasing waste management concerns and its versatility. Even, during

in its mass commercialization is cost and the lack of nations envision it to have a huge potential in their • Aanhurdle • Many infrastructure. future energy mixes to achieve their energy targets with low carbon emissions. Proper state policies and legislation in there is stiff competition in its usage in the • Currently, regards to bioenergy is the only way to make this source transport and agriculture sector. mainstream. Such policies are evident in Scandinavia, used as co-fired or as a replacement to fossil fuels in • Usually especially Denmark, as explained in Section 4.2. CHP plants, without optimum performance. versatile range of bioenergy sources makes each fuel type, Studies to compare bioenergy usage in different energy • Apredefined • to a certain set of applications, with best sectors have been published. It is concluded that greenhouse

Challenges

technology to implement this idea exists, and in fact is of infrastructure to connect industrial waste heat to DH • The • Lack • mature. grids. only way to transform this concept into large scale variation on quality of waste heat due to different • The • Extreme practical setups is by long-term policies and infrastructure industrial segments. development. Hence several policy papers and feasibility of demand and supply along with their • Mismatch • studies have been published. topographies. most industrial waste heat is harnessed and • Currently utilized within industrial setups ranging from a magnitude • of a few KWh to a potential of many GWhs.

•

•

•

biogas is a developed technology to be used on a larger scale DH network. Most research and studies focus on techno-economic analysis for substituting this renewable source, from conventional fossil fuels. Large scale combustion mechanisms and mobility studies are also an important aspect with respect to bioenergy. Used both in small scale rural setups and industrial thermal plants. As a pilot project, a biofuel village has been introduced in Lower Saxony, Germany, for the cogeneration of heat and electricity. An optimization model based on linear mathematics is used to match the production and demand in an extremely reliable and optimized manner. Large scale setups are visible in many DH networks in Sweden (Section 4.1). Biomass fired heat generation sources are available in an array of sizes even in large scale setups to micro setups. They are also used in hybrid with conventional fossil fuels. Typical generation sizes vary from 0.1 to 50 MW.

pellets and chips have been the oldest source for • Wood heating, especially in rural areas. Biofuels and especially

Overview

Table 1 Prominent renewable and novel energy efficient generation technologies for DH.

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Air source absorption heat pump TRL-3

Seawater source heat pump TRL-6

Geothermal TRL-9

Solar thermal TRL-9

Technology

Table 1 (continued)

strategies are inevitable.

improvements could enhance the heat generated • Efficiency especially in regions with low solar irradiation.

immature technology in the research phases • Relatively theoretical studies or experimental work. • Only studies are parametric using simulation tools to • Most analyze the different variables in conjunction to DH.

conjunction with a DH during the cold winter season to provide heating to a pilot project.

technology in the research and field-study • Early-stage phases. and potential to use at a large scale. • Applicability a field study, measurements of an actual setup in • Innorthern China are analyzed. The pumps are used in

•

using geothermal heat source in hybrid with new technologies. Many nations have already harnessed their potential to the maximum. The technology is on a small to medium scale with most setups ranging from 2 to 50 MW thermal energy. In Europe, the French and the Germans have the highest number of geothermal setups (Section 4). On a small scale GSHPs can be categorized as geothermal sources since they harness heat at a shallow depth below the earth's surface.

•

•

•

•

dependent on the outdoor air conditions, making it • Highly • unreliable. on a small-scale or decentralized arrangement. • Used • The overall economic viability and efficiencies are quite low. •

applicable near coastal cities. • Only and energy intensive due to the number of pumps and • Cost annual maintenance requirements.

investment costs due to the drilling and equipment • Heavy required, making it economically uncompetitive.

losses.

available at remote areas, requiring the heat to be • Usually transmitted to urban centres, which eventually increases

maximum.

contamination to indoor central heating systems of buildings.

temperatures involved can get considerably low, • The • requiring 4th generation grids to harness the potential to the

•

•

•

[44,46,47]

[43–45]

[40–42]

[36–39]

Ref.

(continued on next page)

summer the heat from waste can be used to generate cooling in DC, with a COP of 0.50 Storage medium used in most studies and commercial projects is hot water. However extensive funding and studies are focused on high density latent heat storage, in conjunction with DH. Several pilot projects and research cases are being implemented based on 4th generation DH with solar thermal heating, as explained in Section 2.3.1. The potential for usage in the future is high, especially in small scale decentralized setups. Several detailed studies are being carried out to use solar thermal systems with seasonal storage with hybrid technologies to bridge the unpredictable gap between supply and demand. Numerous studies aimed on providing optimization algorithms to develop the best sizing methods for solar thermal, in DH grids have been developed. Solar thermal heating can reliably provide 30–90% of the demand for conventional houses, between winter and summer respectively. The exergetic efficiency of a conventional geothermal system is about 25%. The highest improvement potential being in heat exchangers and the water cleaning process. To harness the maximum potential in a DH grid, a low temperature, supply with 50 °C and a return temperature of 40 °C, was researched upon. Recent advancements in 4th generation grids would make this potential, a possibility, as detailed in Section 2.3.1. Once again high density latent heat storage can decouple the mismatch between demand and supply over geographic coordinates. To make geothermal heat sources economically viable several hybrid combinations and different extraction techniques have been devised. Instead of extracting heat directly, an absorption heat pump to extract low temperature heat from geothermal water of 19.5 °C can be used to enhance exergetic efficiencies by about 13%. The relatively constant temperature of the sea ensures good performance throughout the year, making it possible to transmit heat over longer distances. Results of the pilot study in China showed that the energy consumption of the pumps alone was 79% of the total system consumption with a low COP of 2.43. Hence it is uneconomical to compete with conventional heating technologies. A small temperature drop leads to a variation in pump performance hence unsuitable for long distances but perfect for shorter distances, with lower performance fluctuations only due to the external environmental conditions. Suitable only for temperatures above sub-zero. Usually a decentralized low temperature heat source, requiring the usage of 4th generation DH. The potential for usage with excess electricity production, in small decentralized setups is huge.

Prospects

thermal energy is unpredictable and not a reliable • Solar • option without large scale thermal storage. It is a low-grade heat source; hence the requirement of a low • temperature 4th generation grid is necessary. decentralized solar thermal generation at small scale • For • setups, 4th generation grids along with enhanced control

Challenges

eldest and most mature of all DH technologies. Most heat sources are low in efficiency, partly since • The • Geothermal research aimed at improving exergetic efficiencies and they are used in indirect heating because of the chances of

Bavarian Research Foundation in Germany implements a dynamic optimization model ‘deco’ for the analysis of solar thermal heat with seasonal storage, after testing it on 100 passive houses in a pilot project. At the same time the largest solar thermal DH plant was inaugurated in Silkeborg in December 2016, with a capacity of 110 MW thermal energy.

mature technology, with most research aimed at • Aimproving efficiencies and incorporating heat storage. on a small to medium scale. Can be both • Incorporated centralized and decentralized. research and commercial models have been • Several demonstrated. A recent experimental analysis by the

Overview

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Distributed absorption heat pumps TRL-2

Technology

Table 1 (continued) Challenges

an indirect heat exchanger the operation of a pump is analyzed. The driving force of this pump is the temperature difference.

a theoretical and conceptual ideology. investment costs and system modification required. • Only • Higher in any DH network (Section 2.3.1). Higher maintenance and long payback time on investment. • Used • at the node where the primary heat of the DH • Operates grid is transferred to the secondary grid. Instead of using

Overview

•

• •

evaporators and compression assisted pumps. Efficiency improvements and reductions in maintenance costs will eventually make these pumps feasible, in the future heat mix. Compared to conventional setups about 23–46% of energy can be saved. Unsuitable for lower temperature grids especially future 4th generation grids, since the driving force itself ‘temperature’ is lower. Only suitable to revamp old DH networks especially in Eastern Europe as described in Section 4.

different manipulations into the pump design are • Several carried out including cascaded heat-exchangers as

Prospects

[48–50]

Ref.

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426

Fuel-Cell CHPs

Organic Rankine Cycle (ORC) - CHP

CHP-Hybrid systems

Combined Cycle Gas Turbine (CCGT)CHP

Gas-fired CHP

Coal-fired CHP

Biomass CHP

Technology

• • • • •

•

• •

• • • • • • • • • •

•

range of 1–15 kW normally use wood pellets or chips, for domestic/commercial buildings. Most research focuses on the techno-economic aspects to integrate biofuels in existent CHP plants, to reduce the carbon footprint. The oldest and most mature technology used since the 1980s Only large scale plants exist with prominent sizes being in the range of 300–600 MW. Most studies focus on operational issues to enhance efficiencies. A developed, mature and reliable technology considered environmentally friendly. Usage in both large-scale CHP systems and mCHP setups. Most studies focus on developing alternative sources of fuel especially syngas or biogas. A mature, developed technology capable of having variable fuel sources. Cascaded operation in which heat is generated in the third phase after a Brayton and Rankine cycle, making it highly efficient. Used on a large scale with typical sizes greater than 600 MW. Research focused on optimum performance and ratios enhancements of energy produced in the different thermodynamic cycles. Relatively a new method, with the influx of new heating technologies Used both on a large and small scale with several studies on pilot projects e.g. CHP with solar thermal and sensible energy storage has been implemented in a pilot study A mature thermodynamic cycle used for low heat sources especially biofuels, waste heat, solar thermal energy and geothermal sources. Usually used on a small scale typically a few kWs in size. Most studies focus on using different fuels to visualize the probable performance. A mature, commercially available technology Usually used at a micro level or small scale setups Studies focused on techno-economic analysis of integrating such systems in building and district levels. Performance studies are also performed with Germany being the leader in this technology (Section 4.4.1)

recent development to use biofuels in CHP plants. • ACanrelatively in large scale plants either completely or by co• firing,betoused reduce carbon emissions. Micro-CHP systems in the

Overview

Table 2 Prominent CHP generation technologies for DH. Prospects

published.

will become obsolete due to the high carbon emissions, • Probably low efficiencies and high investment/maintenance costs

of an infrastructure to provide hydrogen. • Lack production is not as competitive as • Hydrogen compared to fuels studies on integrating fuel cells for the • Limited cogeneration of electricity and heat, in DH grids.

only suitable for limited small scale, low • Typically, temperature applications. fluid, choices being limited with the • Working recent ban on CFCs

investment costs • High • More complex control strategies and constraints

a decentralized level.

fact that it is a clean source of energy, with national policies • The on the way, the future seems optimistic. is under way, to analyze the operation on domestic • Research operating conditions with both heat and electricity production, at

[60]

[14,58,59]

and simulation tools are paving the way for easier and • Research enhanced control strategies. technology only for decentralized low-temperature • A4thpromising generation grids of the future. the limited choice of working fluids will probably make • However, this technology obsolete soon.

making such combinations highly desirable.

[57]

[56]

[55]

[51]

[54]

Ref.

investment costs are higher, the carbon footprint is • Although lower, reliability is enhanced and operational costs are minimal,

framework needed to promote this CHP systems are the best choice both in terms of economic • Regulatory • CCGT technology. and technological factors. Having a huge potential policies and development have been witnessed especially in urban centres of feasible only in extremely dense • Economically Scandinavia (Section 4) urban centres with a high heat demands strategies especially in conjunction to DH Control strategies in complex simulation platforms have been • Control • are quite a hassle proposed to enhance operation, by ongoing research.

(Section 4.1).

alternative gas sources requires considerable technical studies have been published to efficiently • Using • Several retrofitting and investments. integrate alternative gas sources in existent CHP plants to use gas sources with different and policy recommendations to efficiently use • Competition • Techno-economic segments of the energy sector especially transport gas in the existent energy mix e.g. CO2 fines in mainland Europe

carbon emissions • High efficiencies compared with recent • Lower technologies.

tools to implement and analyze economic, • Optimization environmental and energy aspects of biofuel CHP-DH are

leader in biofuel-mCHP systems, as discussed in Section 4.4.

in the last table, the biggest hurdle is use expected especially considering national policies to • Astheexplained • Rampant lack of policies and an infrastructure, along lower the carbon footprint. with competition by other sectors. technologies would be an integral part of decentralized • mCHP heat sources especially in central Europe. Germany is the world

Challenges

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Latent energy storage PCMs

Building materials as thermal storage

Pit storage/geothermal storage – sensible energy

Tanks - sensible storage using water

Technology

Prospects

insulation possible, resulting in higher heat • No • losses, hence only used for unpredictable seasonal demands. • time is very large and control is very • Access low. heating to the required • Sometimes temperature can take years.

•

unfavorable. Hence preferred for small scale storage with a potential of research in insulating tanks. Usually accessible when completely charged or discharged. To enhance control a series of tanks in different arrangements i.e. parallel and series can be used. Pit thermal storage is an option only for long term and peak shaving options. Low control and high access times have limited their use only as back-up options or in hybrid with fluctuating renewable sources.

energy storage density novel higher density fluids are proposed including molten • Low • Many salts, oils, gravel-filled water etc. losses with relatively low efficiencies • High low control, with long charging and investment to isolate and insulate the system minimizes • Relatively • Heavier discharging times losses but at the same time the volume to surface ratio is

Challenges

on performance and storage materials in progress to fully • Research implicate it in future housing standards recent technology with ongoing research progressing at a rapid thermal conductivities of the PCM • Apace. • Low materials. a high storage density, since the supplied heat is used to problems over long-term • Has • Corrosion/decay change the phase of the storage medium. usage small scale pilot projects based on single building-DH in cooling/freezing temperatures • Only • Variations interaction, are being investigated but the potential is huge. Most and phase segregations buildings can store 130–400 kW of power. not stable over the long term, by • Mechanically losing its shape compatible with all tanks and metallic • Not encapsulates

multifamily building in Sweden.

research aims to tackle these issues depending on the • Ongoing circumstances of a situation. are used to enhance the thermal conductivity, usually • Additives metallic granules. and mechanical instability are avoided by taking • Corrosion precautionary measures within the operating parameters. agents and nucleator additives are used, to ensure • Thickening uniform characteristics while in operation.

Europe and Scandinavia are leaders in this storage, with • Central Germany being at the forefront (Section 4). due to the high heat losses and infrastructure • However development costs, associated with it, this technology is limited. research studies are simulation/analytical based to analyze • Most performance. new passive storage concept, using thermal inertia. used for short term storage as a buffer to technologies are low-cost maintenance free, hence with a • Relatively • Only • Passive shave off peaks in daily consumption. thorough insight by research studies a huge potential could be at a small scale, depending on the building architecture. • Used realized. Existing materials including heavy structural concrete can even be or no control, once building is setup. • Little used. Corrosion/decay risks in artificial additives to Ongoing research in utilizing PCM (next row) materials and • • building materials adjusting other parameters, would pave the way for better passive small-scale pilot research studies in Scandinavia have been • Several architectural configurations, for zero carbon buildings. conducted. As an example, analysis was done on a five-floor

pumped into hot rocks deep below the surface of the earth.

3

mature commercially utilized large scale storage mechanism • A(starting at about 50,000 m ). pit storage, water is pumped into an underground cavern or • Inman-made reservoir. In geothermal systems, storage water is

is an old, mature and simple technology, in which heated • This water is stored in a simple insulated drum for short term usage. be used in large scale, in conjunction to DH grids or small• Can scale for internal use in buildings. research focuses on hybrid utilization of this storage with • Most renewable sources.

Overview

Table 3 Prominent thermal storage technologies for DH.

[67]

[66]

[64,65]

[61]

Ref.

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and mostly used only to provide hot water, not domestic heating or DH. Low temperatures and pressures can cause two problems in piping i.e. legionella [26] and moisture [77], which is a major research concern. This recent development of fourth generation grids has the highest overall operating efficiency [51]. Contrary to many previous studies that argue that a lower primary temperature results in higher flow rates and pumping expenses, the overall effect is positive, both in terms of power requirements of the auxiliary equipment and the heat losses. Such grids are versatile and can function in even cold climate conditions [78], to reduce heat losses by at least 5–7%. In current third generation DH, at least two thirds of the initial exergy content in the fuel is destroyed as heat is used only for DH [20]. The annual exergy efficiency of these grids is in the range of 15–18%. However, in the case of fourth generation grids not only is the exergy efficiency higher, but the potential of using low grade heat increases substantially. A summary of the five main advantages of incorporating fourth generation grids are as follows: Fig. 2. Typical load duration curve for a multi-source DH grid.

• Lower energy requirements for space heating and hot water supply with the possibility to integrate both • Recycling of low-grade waste heat and usage of heat from renewable sources, while integrating other decentralized sources • Overall confines to the concept of smart energy grids, with more flexibility and potential to integrate • High efficiency smart low temperature grid with reduced capital

underground pipes running through long distances, without interruption.

2.3.1. Transmission temperatures Based on the supply temperature DH networks can be classified accordingly as in Table 4 [3]: First generation DH was introduced in the USA in the 1880s and was developed till the late 1930s. These networks are the most inefficient; however they still exist in the metropolitan cities of New York and Paris. Second generation systems emerged from the 1930s and dominated the market till the 1970s. Most of the DH in East Europe and the former USSR is still based on this technology, as described in Section 4. Third generation DH emerged in the late 1970s and has been developed to date. Most of the networks throughout the world are based on this technology [26]. Typical supply temperatures are in the range of 70–120 °C while return temperatures are within 40–75 °C [77]. Fourth generation systems are an emerging technology and till date only pilot projects or small scale setups have been initiated, as described in Denmark in Section 4.2.2 [3]. Historically several low temperature geothermal grids have been installed, but not on a large scale

costs

It is quite clear that the trend of DH has evolved into lower transmission temperatures, leaner components and reduced complexities. Some of the most prominent differences, between the first three generations of DH and the fourth generation are in Table 5: 2.3.2. Transmission equipment Accompanying the piping's, a DH grid has circulatory pumps, control equipment and sensors. The pumps maintain a constant pressure in the supply and return piping's, ensuring the water moves smoothly at the desired flow rate. Pumps are also used to overcome the head losses, frictional losses and the elevation required to supply the water to a consumer in a building. Depending on the external conditions, the required energy of the water may vary dynamically, which is why modern pumps in DH use variable speed drives, for enhanced efficiency.

Fig. 3. a) conventional DH topology b) ring DH topology. 428

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Table 4 Classification of DH. Name

Medium and temperature

Advantages

Disadvantages

First generation

Steam > 150 °C

For industrial setups, only

Second generation

Hot temperature water > 120 °C Medium temperature water > 90 °C Low temperature water between 40 and 60 °C

Reliable operation, high flexibility, existing infrastructure

High heat losses, exergetically inefficient, severe accidents possible Exergetically inefficient, unable to integrate low grade renewable sources

Third generation Fourth generation

Lower heat losses, lower capital investment, direct connections to buildings possible, integration of low grade renewable heat sources, exergetically efficient, lower operational costs

referred to as the primary heating side whereas the water and network, in the central heating system of a building are referred to as the secondary side.

Table 5 Difference between generations of DH. First, second and third generations

Fourth generation

Insulated steel pipes Radiator heating systems Fossil fuel based centralized heat sources A lone DH grid

Pre-insulated flexible twin pipes or plastic pipes Floor heating systems Low grade heat, renewable sources with centralized and decentralized integrated sources Integrated smart grid interacting with fuel and electricity grids Overall society initiative

Government or local authority initiative One-way flow of heat

Emerging technology

2.4.1. Direct and indirect connections The interconnection between the DH grid and a building can be accomplished in either an indirect or a direct manner [77]. Indirect connections are the most commonly used, where the primary and secondary heating fluids are separated by means of a heat exchanger. In indirect networks the primary side has the flexibility to operate at any pressure or temperature without a concern of the buildings network. A substation is referred to as the complete equipment used at the interconnection of both sides of the network. It may serve a single dwelling or a building with many flats. A typical setup for an indirect connection, in a dwelling where the central heating system and hot water supply system are separate, is depicted in Fig. 4; Normally control units may also be attached to the substations, to enhance efficiency based on the outdoor temperatures [80]. Usually for a dwelling, the water for space heating is separated from the hot water to make control easier. In direct connections, the primary and the secondary sides are the same, and the DH water comes into direct contact with the central heating system of the building. There is no junction or decoupling interface. The advantage of a direct connection is the elimination of heat losses associated with the interconnecting heat exchanger. Normally in flat topographies, where the static pressure drop is minimal, direct topologies can be used. The capital cost of the controlling equipment and the pressure risk associated with a direct connection are considerable high, due to a higher pressure in the DH grid as compared to that of buildings. Damage, contamination and leakages are common problems in direct connected buildings. Direct connections are normally present in only single dwellings and not entire buildings, to minimize the risks. At the same time, lime is an important compound responsible for clogging of centrally heating pipes in buildings, in this case. Special treatments for hard water are necessary, when supplying a building in a direct manner, to minimize fouling losses. Typically, the pressure of such a direct connection is up to a maximum of 6 bars. Direct systems also operate at a lower temperature as compared to an indirect system, typically 30–50 °C lower [81].

Two-way flow of heat

To control the heat delivered to a customer, the DH grid operator has two variables [77]. Either the flow rate can vary or the temperature. In normal operating conditions both these variables are adjusted, simultaneously. Normally flow changes are used for quicker responses while temperature changes are preferred for long-lasting responses. The greater the flow rate and the greater the supply temperature is, the higher the transmission pressure and heat losses are, respectively. It is the task of operators to find the optimum combination while fulfilling demand with allowable tolerances. It is desirable, for both the grid operator and consumer to have the highest possible temperature differential in supply and return pipes, so that the maximum amount of heat is extracted from the network. To avoid frost damage to the equipment, in the grid, a minimum flow temperature is always maintained in the pipes. The heat losses in a pipe are a major source of loss, as compared to the flow losses. Heat losses are dependent on several factors including the length of transmission, insulation of piping's, external conditions etc. Usually heat losses are in the range of 5–20% of the delivered energy [20]. On the other hand, flow losses are in the range of 100–250 Pa/m [77]. A minimum pressure must be maintained in the network at all points to avoid problems like cavitation. Although there are a number of different piping types and materials available in the market the most common configuration is the two pipe (supply and return) system using insulated steel pipes buried underground, with a life expectancy of about 20–50 years [79]. Several new piping schemes used in markets nowadays include twin-pipes that do not require two separate pipes and even flexible corrugated pipes eliminating the use of joints, enabling them to run several hundred meters. Many new pipes have inbuilt moisture detecting systems, which enhance their life [10]. High temperature and pressure pipes (> 90 °C and 16–25 bars) are normally made from steel, aluminium or copper while low temperature and pressure pipes (< 90 °C and 6 bars) are usually plastic. Polyurethane foam is the usual insulation material and the typical diameter of these pipes is in the range of 25–100 cm.

2.4.2. Heat exchangers The most common types of heat exchangers used in substations, are either the plate heat exchangers or the shell & tube exchangers [81]. Shell & tube exchangers offer a better heat transfer however consume more space and are not as dynamic to control, in events of load shifting. The tubing of most exchangers is stainless steel to avoid fouling damages [10]. In case of larger buildings two or more heat exchangers may be connected in parallel to enhance security of supply. When a large commercial building has different heating requirements it is normal to use many heat exchangers, usually hospitals have this requirement. A summary of the types of, typical heat exchangers used specifically for DH options are in Fig. 5:

2.4. Distribution The water and heating network, in the DH grid are normally 429

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Fig. 4. Typical substation connections for an indirect connection.

flow meter, temperature sensors (normally thermocouples) at both the inlet & outlet and a heat calculator and a programmable calculation unit. Recently, thermal meters having all three main components in one unit have been quite common especially for industrial and DH applications [83]. It is envisioned that future smart meters will allow consumers to view their heat usage in real time, monitor the heat quality, interact with other energy grids including the electricity grid, perform demand side management (DSM) techniques, interact with appliances and even be able to net meter. [85]. Depending on the capital investment and the accuracy desired there is a lot of variety of the types of flow meters in the market [77]. Flow meters can be broadly categorized into static (lesser expensive) and dynamic meters. The most common heat meters are presented in Fig. 6: Over the past years it has been quite easy to distort the calibration of a typical flow meter, being the main reason it is not trusted in most countries (Sections 4.5 and 4.7). Many factors including the length of piping before and after the meter, the position relative to the direction of flow, sediment deposits, air accumulation, sensitivity to flow disturbances etc. have been some of the most common issues.

Once the heat is transferred to the secondary heating fluid, heat emitting radiators are used for space heating. The heat is transferred to the air in the room, at a predefined thermostat setting. Some new buildings, especially multi-floor buildings in China (Section 4.7) have under floor heat emitters, to use the floor concrete as heat storages and minimize losses due to convectional currents within the room. 2.4.3. Heat meters With the widespread use of DH, the issue of heat metering has come under a lot of scrutiny over the past few years, especially in Europe (Section 4.1) [82]. There are two ways to charge a consumer for the heat consumed; either by measuring the heat supplied or by charging a flat rate in terms of the area, of a dwelling as done in many buildings in East Europe and China (Section 4) [3]. The first option has higher capital costs while the second option is not the most efficient to control heat usage [83]. Although charging based on floor area, may seem rational but it increases usage as the sense of consuming energy only when required diminishes, by the end customers. Consequently the Energy Efficiency Directive of the European Commission (Section 4.1) [84], requires all individual consumers to have heat meters and to be billed according to consumption. A typical heat meter is composed of three main components [77]. A

Fig. 5. Typical heat exchangers in DH grids. 430

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Fig. 6. Typical heat meters in DH grids.

3. Social and economic aspects of DH

prospects, to justify such a large investment as the potential is easier to predict. After the establishment of a consistent heat load, the case to install a sustainable source and even expanding the network is made stronger.

Based on a case study in Italy, an exergy analysis was done for the entire energy mix, using a method to compare social-economic aspects with the second law of thermodynamics. It was proved that DH could save substantial amounts of energy in the entire system especially in the residential sector, irrespective of the regional constraints [86].

3.1.2. Transmission The cost of the transmission grid is heavily dependent on the topography served and the heat density of the region. The urban centres in Scandinavia and central European countries, have a lot of multi-family dwellings compared to the rest of the world, which is one of the reasons of lower unit costs [4]. This transmission cost can broadly be subdivided into the civil works required and the mechanical equipment to be installed. Typically, both these costs are almost similar to each other in value. A heat network to supply about 270,000 homes would roughly cost about €1–1.7 billion [88]. In a recent study carried out [91], it was concluded that the excavation costs are highly dependent on the geographical location and economics of a country, based on which there is much disparity between estimated costs. As an estimate the cost of the transmission pipes lies between 450 and 1700 €/m [87]. To minimize the cost of the pipes, it is proposed to use twin-pipes wherever applicable since their performance to price ratio is the best. Another important ingredient in the estimated costs of the transmission network is the experience in construction. In countries with no past record in DH setups, there is no standardized procedure in place, both for the designing of the network and the non-technical formalities. This slows down the entire development process and makes it financially riskier for potential investors. Such high investment is only favored by investors when the rate of return is high.

3.1. Economic aspects Similar to the previous section, the economic aspects of DH are divided into three subsections: 3.1.1. Distribution The operational cost of DH is highly dependent on the scale and operators of the distribution grid. The higher the heat load and density of buildings the more economical it gets in comparison to other technologies [4]. Depending on the utility prices, topography and technical parameters of the grid the operational costs are usually between € 30–40 MWh of energy transmitted [87]. A basic semi-detached three bedroom house approximately needs a substation connection of about 8 kW and is located about 10 m from the main transmission line connection of a DH grid. A distribution of the basic components and their share of the cost [87], of such a building, would be as in Table 6: The capital cost for a consumer installing a distribution grid in such a semi-detached house in the UK, is somewhere between £ 2000 to £ 5000, [88]. Comparatively these costs in the UK are about 20–30% higher than its European neighbors and even higher than many countries in Asia. On the other hand, the costs of a unit KWh of heat from DH in the UK, is in the range of 4.64–9.68 pence [89]. Once again this tariff varies highly per country, depending on the liberalization of the market and government intervention, as presented in Section 4. Initially, when DH is evolving in a country, heat prices need more regulation either by a non-profit operation, cost-based tariffs, marketbased tariffs and/or government subsidies compared to alternative energy utilities. These mechanisms assure that DH is able to compete with the status quo. This is done through government interference and policies within this sector as has happened in East Europe and Scandinavia (Section 4). The classical method by which DH evolved in Scandinavia, was by first developing the heating grid followed by a heating source [90]. As the heat load is gradually built up, the initially installed inexpensive large-scale boiler could be replaced by a more sustainable source. This makes it easier, in terms of the economic

3.1.3. Generation As discussed in the Section 2, the options of generation of heat are quite versatile. Ranging from large scale CHP facilities to decentralized renewable sources, generation is unique in every DH grid around the world, as discussed in Section 4. Since most grids at the moment are third generation with centralized heat sources, a summary of the average costs [72,88] of these common sources in the UK (conversion rate of 1 GBP-1.1Euro), at present are in Table 7: Once again these, prices may vary by about 5–25% depending on the country and technical aspects of the installed units. Heat from waste industrial sources is usually about 10–15% of the unit cost of electricity, requiring transmission pipes from the industrial source to the DH network. It is important to note that for biomass to be economically feasible, both consumption and production must be located close together, which is favorable when using DH networks on a localized level [92]. About 80% of the biomass produced has a potential consumption within a radius of 25 km while 95% can be consumed within 40 km. Thermal energy storage is also considered integral to DH grids, since they act as a buffer, being both a source and supply. Small storage tanks up to 30 m3 cost about €425/m3, while large-scale underground storage of about 75,000 m3 are at €30/m3 [93]. Similarly the construction of an underground pit storage costs approximately 950–1100 €/m3 [87]. Based on a case study of a CHP plant in Sweden [69], the inclusion of a DH grid with the CHP plant, increased the heating demand, which

Table 6 Typical component shares of the prices of a DH distribution grid. Parameter

% share

Heat meters Heat exchanger used as the substation Heat network to building connection Distribution network

14–21 6–9 27–32 41–50

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consumers, the heating sector is one of the most complex and difficult to decarbonize, especially in industrialized economies [21]. However, a recent study shows that if the cost and performance of DH are comparative to the current system, the general public is willing to shift to a more sustainable option, in spite of the difficulties [21]. Another important aspect, of DH is the importance of the indoor environment and fuel poverty. Even in developed countries like the UK about 15% of households are affected by surface condensation and mold growth [90]. This is attributed to insufficient heating, poor ventilation and excessive moisture production in the building. About 11% of households within the UK are suffering from fuel poverty with the main reasons being low incomes, poor efficiencies of heating systems and rising fuel costs [96]. In developing countries even within urban centres these figures escalate exponentially. The current fossil-fuel based heating systems, in typical houses, increases fuel poverty causing insufficient heating with minimal control over the indoor environmental conditions. On the contrary, cheaper, sustainable and wellcontrolled DH is the answer to this problem. In an experimental study in Sheffield [90], the heating system was changed from an electric underfloor heating to a DH installation. Results showed much less mold growth and improved indoor conditions. Fuel poverty results in insufficient heating within dwellings causing lesser warmth. This causes an array of social and economic problems within a locality. The solution is not as simple as warmth cannot be simply purchased. Fuel, appliances and appropriate housing is necessary to eliminate it [75]. Or on the other hand DH is considered as the simplest and most efficient way to reduce fuel poverty especially for the urban poor. Eliminating fuel poverty through DH is important for three reasons:

Table 7 Costs of DH heat sources. Capital cost in €’s Small gas engine CHP 500 KWe Large gas engine CHP 2 MWe Small CCGT - 50 MWe Medium CCGT - 90 MWe Gas DH Boiler Biomass DH Boiler Small Biomass CHP 100 KWe Medium Biomass CHP 8 MWe Large Biomass CHP 30 MWe EfW: Anaerobic Digestion EfW: Combustion Solar thermal Ground source heat pumps Air source heat pumps Electrical heating

Operation and maintenance as a % of capital cost per annum

950/KWe

9.3

723/KWe

7.3

886/KWe 835/KWe 66/KWth 677/KWth 4400/KWe

4.0 4.2 5.0 2.4 4.5

3850/KWe

2.3

1958/KWe

4.5

8520/KWe

10

9625/KWe 1572/KWth 1100/KWth

5.9 0.3 0.9

660/KWth 243/KWth

1.5 5.0

eventually increased the electricity production and base load operation of the plant. This in turn reduced the production costs for both electricity and heat. In terms of economics, environmental and fuel efficiency terms, biomass fuelled large scale CHP systems are the best option for heat generation [24]. Additionally with thermal storage used in conjunction, primary energy consumption can be reduced up to 12%, while total costs can be reduced up to about 5% [18].

• The urban poor spend a major chunk of their incomes on heating. A better heating mechanism would not only provide this basic necessity but also uplift the overall standard of living for the low-class society.

3.2. Social aspects

• Environmentally damaging fuels are usually used as substitutes,

To realize the complete benefits of DH, it must be coupled with public awareness. In an experimental study [94], the energy performance of a low carbon housing society comprising of 25 houses using DH fuelled by biomass (wood-chip boilers), was conducted for a year. A significant observation in this study was that although the overall carbon footprint of the system decreased, there was a wide disparity in the actual heat demand of the consumers even with those having similar characteristics e.g. floor size, occupants etc. The concluding factor is that despite having sustainable heat generation sources, to reduce the overall carbon footprint the heat demand is also to be minimized, which is strongly linked to consumer behavior and maturity. Cheng et al. [95], developed the Domestic Energy and Carbon Model (DECM), to predict the future energy consumption and carbon dioxide production, of the UK residential sector. The model was validated with national statistics with only marginal errors, making it a valuable tool for holistic consumption predictions of all household utilities. Results show that 85% of the variance in energy consumptions is dependent on the dwelling type and socio-economic conditions of the dwelling, making public awareness an essential part. The issue of, greenhouse gas emissions and the human impact on climate, is one of the sole driving forces of public awareness into more sustainable heating mechanisms like DH. With DH, the need for individual fuel suppliers, boilers, flue gas treating technologies and storage of potentially hazardous fuels is eliminated from built environments. In densely populated residential areas, DH not only makes more economic sense but is also safer and reliable [90]. At a domestic level, heat reduction can be done through demand-side management techniques. With DH the possibility of installing smart meters so that consumers can make better decisions, with real-time data to efficiently consume energy, is easily possible. Due to the wide range of heating applications in industries and domestic

•

including coal and waste, which are usually combusted in the poorest conditions. This results in extreme environmental damage, as was witnessed over the last decades in the urban poor areas of China. This often leads to short-term solutions damaging infrastructure and city-planning, on a whole.

A balance between policies by the central government and local initiatives by city councils is the key ingredient for the success of a DH network [97]. From the conception, construction to the operational stage of a DH grid, social participation is of utmost importance. Private financing, of low carbon technologies, through local citizens is an interesting option suitable for the conception of DH in liberalized markets of today [98]. Although government intervention in the form of quota schemes, soft loans, tax incentives etc. are needed; such a financial mechanism reduces the risk on individual investors and generates the necessary momentum in the society in favour of DH. In many European states, the city councils have much stronger administrative responsibilities and financial muscle [99]. These city utility firms, formulate their own policies and operate the overall energy mix to the advantage of the locality instead of monetary gain [100]. This ensures that the advantage is always towards the society and not the firms. At the same time this strategy; helps develop local manpower and technical expertise. 4. Status and policies of DH grids, worldwide Sustainable energy growth has evolved as a pressing concern all over the world. Over the years several summits and conferences, with the aim of addressing global issues concerned with energy by formulating policies, have been in the limelight. The important 432

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Table 9 that the largest DH grids in central Europe are in Poland and Germany. A summary of the statistics [62], of some of the most prominent grids is presented in Table 9:

Table 8 Country wise classification of DH networks. Grid status

Countries

Emerging Expanding Consolidating Refurbishing

Canada, USA, UK China, South Korea, Germany, Italy Austria, Denmark, Finland, Sweden, France Bosnia & Herzegovina, Croatia, Kazakhstan, Kosovo, Macedonia, Russia, Serbia, Ukraine, Uzbekistan, Bulgaria, Croatia, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania, Slovenia, Slovakia

4.1.2. Policies and outlook The European Union (EU) passed four important resolutions in its parliament over the last couple of years, linked to DH. In 2009 the European Parliament presented the daring Renewable Energy directive [104]. As part of this directive, by 2030, renewable sources would generate 30% of the final energy and greenhouse gas emissions would be reduced by 80–95% of the 1990 levels in 2050 [105]. With a focus on sustainability and a low carbon future, the Energy performance of buildings (EPBD) [82] was introduced and consequently the Energy Efficiency Plan (EED), [84] was put forward. The overall theme of these policies in terms of heating is to have a sustainable and carbon free system by 2050 which promotes DH, to urban buildings. These directives provide clear guidelines, of the policies and responsibilities of member EU states in imposing such system. According to the EPBD all future buildings are required to be zero-energy or near zero-energy, and in the latter case must only be energized by renewable technologies. According to the EED, energy efficient heating, can only be achieved through efficient co-generation technologies (Section 2.1.2) supplied via DH and DC systems. Co-generation technologies supplying the grids must have at least 50% of the energy source from renewable energy. As per stats published by the European Technology Platform the potential of DH in Europe is huge. It is expected to supply at least 25% of the heat consumed by 2020 which may go up to about 50% by the year 2030 [1].

requirements for most energy systems in the future are:

• A sustainable system to meet the end user demand (secured energy) • An optimized system having the least operating cost (competitiveness) • A low-carbon system (renewable sources) Every nation has planned to curb carbon emissions and since the heating sector is a major source of fuel consumption, policies to revamp this sector are in place. DH is the most viable option, which fulfills these three important requirements of a future heating system. Over the last 50 years DH has evolved from a steam carrying network in industrial units to a comprehensive low carbon heating solution for both domestic and non-domestic buildings (Section 2.3.1). The fact that in only China and Russia, DH supplies heat to more than 200 m people, is proof of its importance and the extent it has progressed [101]. Countries can be broadly classified in four groups [3], per their DH usage as summarized in Table 8: The specific topographies, in terms of the different segments described in Section 2, and policies of unique leaders in this regard are presented.

4.2. DH grid status and policies in Denmark 4.2.1. DH grid status At the moment Denmark has an estimated 285 decentralized CHP plants, 16 large scale centralized CHP plants and 130 plants with backup boilers [102]. The DH grid meets 46% of the heating demand of the entire country. Decentralized CHP producers are spread throughout the country, injecting about 20PJ of heat into the grid annually. In 2004, the CHP-DH network saved an estimated 8–11 million tonnes of CO2. Denmark is also considered to be the pioneer in long term thermal pit storage, with several examples throughout their DH network [39]. Denmark also integrates heat pumps interconnected with wind farms to shave off peak electricity production patterns. The most famous DH network is in Copenhagen. The energy grids in this city are probably the best in the world which have successfully integrated power, district heating, natural gas, district cooling and waste management, into a combined system [20]. The DH grid provides heat to an estimated 270,000 households since 1984. The sources of fuel for the CHP plants in the municipalities of Copenhagen vary from fossil fuels to biomass along with having a geothermal source, serving 90% of the population of this city. The length of the transmission network is a web of about 54 km, serving water at 120 °C and a 25-bar pressure. Svendsen et al. [106] conducted an analytical study of the Copenhagen DH grid to conclude that building refurbishment is necessary for the next step in implementation of low temperature 4th generation grids (Section 2.3.1).

4.1. DH grid status and policies in the European Union (EU) 4.1.1. DH grid status As evident from, most grids in Europe are comparatively well developed. The grids in East Europe and Scandinavia are prime examples of Europe's long association with DH. In central Europe, the grids are emerging and most importantly, have sustainable heating sources (Section 2.1.1). Although former Soviet Union states were much more developed in terms of a DH infrastructure, it is the Scandinavian countries that are using energy efficient fuels to generate this heat. The development of such an extensive DH grid in these USSR states is because the demand of heat is extremely high due to the harsh climate. In fact, in some Balkan countries there is a greater demand for heat as compared to electricity, which is quite astonishing. Thus, some of these nations have, a surprisingly, more through heat network in comparison to an electricity network. It was not until the 1970 oil crisis in which the price of oil was inflated by the OPEC that Scandinavia realized the use of alternative fuels for their heating industry. Consequently, Denmark has become a world leader in this technology and based on its low-carbon sustainable model, its DH is acknowledged throughout the world. More than 75% of the heat used in DH networks in central Europe are from low carbon sources [102]. Iceland has the highest number of citizens connected to DH with an abundant source of geothermal energy (Section 2.1.1), which it quite efficiently connected to the DH grid, over the last 50 years. It has a relatively low population with most people concentrated in a handful of cities, making DH easily accessible and cheap. Some DH grids in Europe are highly sustainable; others are extremely innovative while some serve in harshest of climates. Every DH has its own uniqueness and has evolved through different conditions and policies. It is evident from

4.2.2. Policies and outlook After the devastating effects on the heating sector in the 1970 oil crisis, the state initiated a strong national regulation to promote sustainable DH grids. Based on this movement Denmark aims to be completely independent of fossil fuels by the year 2050, as per the EU directive in Section 4.1.2 [15]. Most of the generation plants are owned by local authorities and with such a high dependence on DH, prices are highly regulated. The 433

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Table 9 European DH stats for 2015 [103]. Parameters

Unit

Denmark

Finland

France

Germany

Iceland

Poland

UK

Sales Sales Citizens Length Number Capacity CHP share

TJ mEuro % km

105,563 2945 63 29,000 394 – 73

114,160 1861 50 13,850 400 23,270 73

86,112 1634 7 3725 501 21,230 23

254,839 5701 12 20,219 3372 49,691 81

28,181 145 92 – 48 2290 –

248,693 3083 53 20,139 317 56,521 57

– 437.5 2 361 2000 335 80

MWth %

Table 10 Advantages of DH systems. Category

Advantages

Consumer

Space savings in the building Noise-free operation, with no ventilation or fire protection required Easy to control and operate comparative to conventional systems Price per unit energy is lower and stable due economies of scale Better management with enhanced room comfort Extremely low capital costs compared with conventional systems More reliable with enhanced supply security, several backups and a thorough transmission grid ensures supply even with a failure in the system [2] Safer buildings with no flammable fuels and flue gases More flexible sources in terms of the waste heat utilization, renewable sources, more efficient thermal conversions in CHP and the associated efficiencies with economies of scale for heat-only thermal plants [21] Capable of covering very large areas and an array of building types at once Flexibility in operations makes it relatively easy to control once implemented With low operational costs the rates of returns on the capital can be quite good Sustainable in the long term with low maintenance costs and lifetimes of 20–50 years Environmentally friendly with carbon reductions [22] Heat metering techniques ensure energy is efficiently used Capable of incorporating a range of renewable low carbon technologies including the most common biomass fired CHP stations [2] Normally is a collective effort from councils and locals resulting in efficiently managed systems More efficient in terms of energy and exergy usage of fuel based systems [25] Can provide cooling when there is no demand for heat through large scale absorption refrigeration cycles [35]. Thermal storage in DH grids is the cheapest form of energy storage at the moment

Distributor

General

Table 11 Summary of DH status of different countries. Country

DH status

Policies

Denmark Finland Germany Russia Poland China USA

Most updated, sustainable and best operated DH network of the world One of the oldest having versatile sources of generation Unique, ambitious, sustainable and advanced networks of the world Largest, eldest and inefficient network in the world Diverse networks in terms of sources, sizes and operation Fastest growing network in the world Mostly small private networks providing both hot and cold water

High government involvement in legislation and operation Minimal government involvement, evolution due to dire need Extremely aggressive government involvement Mostly outdated state owned, need for revamping Government involvement for sustainable transformation needed Government involvement used to reduce carbon footprint Increasing government support, esp. for renewables recently

with a supply temperature of 55 °C and return temperature of 25 °C. The operation was reliable and different configurations for the booster pumps, network topologies and flow conditions, based on the different constraints of the year, were implemented and analyzed in detail, including ring topologies (Section 2.2).

next target of Denmark is to substitute biomass in the already operating fossil-fuelled CHP plants to achieve this transition. At the same time the Danish Board of District Heating is helping many other countries including China (Section 4.7) to setup similar grids. It is important to note that although DH is favorable for bulk transportation of heat, it is not quite the best option compared to heat pumps for less dense areas, even in Denmark [107]. Lund et al. [15] developed a simulation for the scenario analysis of the most efficient heating shares in the Danish market in 2060, to find out that the share of heat pumps is considerable high and DH can only penetrate 65–70% of the market in the future. A major reason for this is the production of excess electricity by the various wind farms in the Danish market. This is the first problem of this type considered in Europe. The coverage of DH in Denmark is almost at its maxima, however future studies are paving the way towards low temperature more efficient sustainable technologies, in trendsetting Denmark [108]. A case study [81], in Trekroner Denmark was carried out to study in depth the characteristics of a low-temperature fourth generation DH grid (Section 2.3.1). This network was about 1.4 km to serve about 165 households

4.3. DH grid status and policies in Finland 4.3.1. DH grid status DH networks originated in Finland in the early 1950s [109]. In fact, despite the DH prices being low and minimal government support, investors have had a great rate of return owing to it being an essential necessity. There are about 150 DH companies operating in Finland and about 200 municipalities in the country use DH grids. CHP plants are the most prominent source of heat for the DH grids, delivering about 13.5 TWh annually. About 20% of the fuel in these plants is renewable, and the total demand of heating is about 37 TWh annually. The most notable DH in Finland is in Helsinki which serves 92% of the buildings in this region. It produces about 8 TWh of heat annually 434

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of DH to industries themselves is essential to fulfil its environmental objectives [83]. A variety of simulation studies have been carried out to act as the pivotal tool for most of Germanys ambitious policies [113]. The differentiating fact of German policies compared to Scandinavia or other European countries (Section 4.1.1), is the fact that Germany is a strong advocate of decentralized small scale heating sources connected to a regional grid. State policies even favour excess electricity driven heat pumps. The overall aim is to create a flexible grid with enhanced security and reliability by making it least dependent on a central heating source. Although the idea seems quite practical and intuitive, the investments required for such a system are considerably high and at the same time more control is necessary for operation (Section 2.2).

mostly from CHP plants which also generate 5 TWh of electricity. Due to the climate conditions, space heating demand in Helsinki exists all year around and a third of the heat produced is used for hot water. There are approximately 114,500 building connections to the DH grid in Helsinki. Unlike Denmark, there are few small scale CHP plants in Finland [3] 4.3.2. Policies and outlook Like Denmark the Finnish government aims to be fossil-fuel free by 2050 (EU directive Section 3.1.2). Contrary to Denmark, the government has lesser interference even for CHP development [16], but the fact that it is an extremely cold country with minimal natural resources was in itself enough to drive it towards CHP-DH. Finland is facing challenges in exploiting its remaining CHP potential and getting more coverage of DH grids. Most of the centralized heat producing plants are fossil fuel based either using heavy fuel oil or natural gas, causing fluctuation in prices. There is a considerable potential to use wood or biomass as a source of fuel (Section 2.1.1) [109]. The inclusion of biogas is only possible at large scales and thus is the most convenient way for Finland to revamp its DH network to make it affordable again [110]. However, the infrastructure of using wood and biogas sources is still in its infant days making the security of procuring this fuel a major hurdle for operators [111]. A rational is that the use of renewable heating sources is a combination of social-technical considerations at a local level. However, the shape of the considerations and the momentum with which localities use renewables is defined on a national scale. Finland is lacking a national roadmap of the future of its heating, which with the passage of time is becoming more essential. It is expected that heat pumps will play a vital role in the future too, especially with the presence of unpredictable excess electricity by wind farms, in rural areas [112]. There has never been a heat law and most commodities are considered as commercial products, up till now [16].

4.5. DH grid status and policies in Russia 4.5.1. DH grid status There are about 15,000 DH operators countrywide. There are also, about 500 large scale CHP plants, and 65,000 large scale boilers [102]. Approximately half of the DH grids are supplied by CHP plants while the other half by centralized boilers. The main fuel in these heating plants is gas with coal being the secondary source (Section 2.1.2). The total estimated length of the DH is about 200,000 km delivering about 1700 TWh worth of heat. 30% of this heat is produced by CHP, 45% by boilers and the remaining is industrial waste heat. DH covers about 70–80% of the country's heating demand serving approximately 70% of the population [114]. The production of heat in the DH networks are mostly either private sector enterprises, local industrial producers or municipalities [114]. The transmission grid connecting the generation to the users is mostly state owned. The DH grid supplies both residential users (45%) and industrial consumers (38%) along with commercial, agricultural and public sectors. Moscow's CHP-DH network is considered as the largest and oldest in the world [101]. It has 15 large CHPs, 170 heat-only boilers, connected to DH grids about 8000 km long. In 2007, this network alone delivered 120 TWh worth of energy to a range of consumer classes.

4.4. DH grid status and policies in Germany 4.4.1. DH grid status The DH market in Germany fulfils 14% of the space heating demands in the country and by 2020 it is expected to have 25% of electricity generated via CHP stations. In 2005 there were a total of 2938 CHP locations with an installed electric capacity of about 20 GW. At the same time Germany aims to become a regional leader in mCHP units with intense legislation in this area, with technical details in Table 2 of Section 2.1.2. The Saar District Heating Network is one of the eldest and comprehensive ones in the western county of Saarland. The city Saarbrücken is the centre of this DH scheme serving about 13,000 consumers of all sorts. A 680 MW CHP station is one of the main sources of heat. This grid was developed in 1976 with an investment of about €250 m. The fact that most of the heat is either industrial waste or biomass CHP, makes it one of the most sustainable in the region [102]

4.5.2. Policies and outlook Russia has the longest and deepest association with DH, since the last 100 years (Section 4.1.1). The main drivers of this ancient DH grid were the socialist policies before 1990 and the climate [115]. The communist ideology resulted in minimal investment in this sector, causing an inefficient DH throughout the former USSR. Abundant fossil fuels were the main drivers for a boiler based DH grid. There has been no major policy in terms of sustainability or carbon reductions thereafter [3,102]. The only major noteworthy policy has been the Russian Energy Efficiency strategy of 2030 which aims to reduce heat losses in DH grids from 20% to 10% by 2030. Heat metering is not completely implemented and consumption based billing with heat meters is uncommon, adding to the inefficiency (Section 2.4.3). The biggest challenge for Russia is to transform this aging DH network with more sustainable systems. Over the recent years the network has become obsolete yet is still socially important. Most users are shifting towards other decentralized heat sources to enhance their security of supply, during the peak seasons. Critics [114] believe that unless major overhauling, initiated by the government does not occur, only the poor or urban centres would sustain the burden of this system. Several government initiatives including installing meters and increasing prices to generate capital have been implemented over the years but until major overhauling is not done, the downfall of this system is likely to continue. It is estimated that over 60% of the DH network is in need of major repair, with some pipelines being more than 50 years old [116]. Normally in many European states transmission losses are in the range of 5–10% while in Russia losses as high as 50% have been recorded. For this reason, innovative technologies, trained professionals and state policies must be implemented. Another challenge is to introduce competition in these mostly state-owned networks

4.4.2. Policies and outlook Germany has one of the most ambitious and aggressive energy policies in Western Europe. Among these reforms is the most famous ‘Energiewende’ which aims at reducing greenhouse gas emissions by 80% of 1990 levels and having a 100% renewable energy system by 2050 (EU directive Section 4.1.2) [23]. The state also regulates CHP and DH laws, with an expectation to double CHP capacity by 2020, through financial incentives [83]. Innovatively, Germany is one of the countries in the region that has specific laws for fuel cell CHPs (Section 2.1.2). Since Germany is Europe's economic hub and pivotal region in terms of gas, heat and electricity networks, these reforms will have far reaching implications throughout central Europe. Most of the DH networks in Germany are operated by city administrations. This ensures regularized unit prices and smooth expansion of the current facilities. However, being an industrial state, the expansion 435

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demand annually which is about 18% of the overall demand. In Northern China, DH alone accounts for 63% of the heat demand. The DH grid in China are widely scattered with only Beijing having 5000 operators. In 2005 CHP plants supplied about 60% of the required heat in Beijing. By 2005 there were about 329 cities in China having DH networks supplying over 2000 PJ of heat. China has approximately 85,000 km worth of DH pipelines scattered throughout the country. Since coal is cheaply available in China most of the DH uses this source as heat, but it is expected to be replaced by renewable sources, in the future.

to make it more vibrant and improved. In simple, sector wide reforms from generation to distribution ends are required, with active financial support by the government [117]. 4.6. DH grid status and policies in Poland 4.6.1. DH grid status About 40% of Poland's population is served by DH with about 20,000 km worth of pipelines. There are about 460 DH operators in the country which are both state and privately owned. All the prices are government regulated along with development policies. 62% of the heat is coming from CHP plants equalling about 162 TWh annually. Poland has the most diverse network profile with a range of sizes, sources and operating principles in different parts of the country. Currently, coal and gas are the main sources of heat generation. The largest DH network in Poland is in Warsaw and is also considered as the largest in central Europe. This network covers 80% of the city's demand, by producing about 11 TWh annually having 1600 km of installed pipelines [102]. The sources include CHP and large scale boilers.

4.7.2. Policies and outlook By 2020 China plans to have a 15% renewable energy share in its energy grid and cut carbon emissions by 40–45% of the 2005 values. Based on the outcome of a simulation study, a thorough DH grid in China could cut energy consumption for heating by about 60% while decreasing the heating cost by 15% of the current levels [120]. Hence inarguable to reduce consumption, carbon emissions and satisfy the needs of such an accelerating economy DH is the most viable option [120]. Greenhouse gas emissions are the biggest challenge for China. According to the World Health Organization (WHO) about 70% of people living in urban centres are breathing hazardous air. This being the reason that China plans to convert most of its urban DH grids to gas sourced heating instead of coal based [122]. Most of the coal based CHP plants in China have a steam extraction ratio from the turbine of between 0.25 and 0.6 [123]. With the conversion to gas CHPs, this ratio can change without severe implications depending on the load, making it a feasible option (Section 2.1.2). One of the biggest areas where the DH grid in China is non-competitive is the fact that heat meters are not common at either consumer or supply side and in fact generally there is a lack of control and monitoring equipment (Section 2.4.3) [121]. This leads to slow response times, less flexibility and in general much more heat wasted than equivalent setups in European states (Section 2.2) [122]. Although the will is present and policies are in place to develop the DH grid into a sustainable network, the transition is expected to be extremely slow considering the booming economy. In the years 2010–2012 one third of all research papers in the world on DH originated from China [124]. Most of the research is focused on expanding heat sources and better controlling of DH grids to enhance the overall advantages. This shows the pace at which China is progressing to secure a sustainable heat supply.

4.6.2. Policies and outlook The harsh winter season, incentives from the EU (Section 4.1.2) for the development of CHP-DH grids and the availability of Russian gas, have transformed Poland into a country with one of the most comprehensive DH networks in the region. Keeping this in mind, it is evident that the heating network is not the most sustainable and environmental friendly [118]. Presently most policies of the government aim at improving efficiency, modernization of the grid, reducing emissions and promoting CHP plants instead of conventional large-scale boilers. Upgrading of the Polish infrastructure is a capital-intensive process, which so far has not been easy. Revamping of the Polish system is a two phase process [119]. In the first phase heat generation sources, must be diversified to sustainable sources. This includes biomass, waste industrial heat and solar thermal amongst the most prominent available sources. In the second phase the distribution end must be upgraded. This includes installing heat meters in all the dwellings, upgrading the central heating system of old buildings including the radiators and finally retrofitting buildings to reduce the overall space heating demand (Section 2.4.3). A challenge facing DH operators is keeping the prices competitive while renovating the entire system, if unsuccessful in this objective; the path for many decentralized heat sources will be open. Although the task is quite hard but the fact that Poland has no other viable option and DH is a necessity will be the catalyst, to push to, modernize this old-fashioned system.

4.8. DH grid status and policies in United States of America

4.7. DH grid status and policies in China

4.8.1. DH grid status There are about 600 DH schemes in the US, with about 16.6 GW of installed thermal capacity. 106 of these schemes are in city centres of which 55 have CHP-DH networks while the remaining are found in university campuses and hospital complexes. Additionally, there is also about 5 GW thermal of DC capacity installed. About 1.3% of all commercial buildings are connected to either a local DH grid or a larger city-wide grid. In the US, the operation of DHC is throughout the year, which makes more economic sense. Unlike European nations DH, mostly serves commercial instead of residential buildings which are usually located in clusters. At the moment, the US has an installed CHP capacity of about 82.4 GW, representing about 4200 medium and large sized facilities. Most of the capacity, over 80%, relates to the industrial sector. Only 6.6 GW of thermal energy is supplied to DH by these CHPs. Most of these plants are natural gas fuelled and the development of new facilities is at a favourably impressive rate. Especially with the recently explored shale gas reserves, interest in developing gas-fired CHP stations has risen. Some notable DH schemes are found in Indianapolis and

4.7.1. DH grid status China has the highest growth rate of DH grids since 1998 with an annual growth of about 10–15%. Like China's economy, the growth rate in DH has been exponential and is expected that the served building area will rise to about 7000 million square meter, by 2020 [120]. This expansion can primarily be associated with two main reasons; population and pollution. Most of China's population has been moving into densely populated urban centres, especially in the cold areas of the North, which is best supplied by DH [121]. On the other hand, in-house boilers have considerably contributed to pollution and emissions problems in urban centres resulting in enhanced DH grids through major city centres. Consequently the government is promoting clean energy policies especially in the field of CHP-DH networks, by subsidizing unit prices in most municipalities [3]. It is not only DH that is booming in China, CHP plants have seen an even greater rise. CHPs cover about 30% of the DH supplies and are expected to double by 2020. China has the world's second largest installed capacity of CHP plants. CHP plants feed in 2300 PJ in the heat 436

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• Have multiple sustainable centralized and decentralized sources; • Any building can be connected and operate either a source or sink in different time domains; • Bi-directional energy flows with appropriate heat metering techniques; • Controls to remove the barriers of location and time in heat supply; • Communication and interaction with electricity, gas and transport

Minneapolis, serving about 200 buildings, with additional DC capabilities. The DH scheme in Massachusetts is also quite famous, and serves about 250 buildings, including 70% of the office towers in the city centre and the top-ranked universities. This DHC service, has been operating in this region for several years, with approximately 381 Mt of steam produced annually from CHP stations which reduces CO2 emissions considerably. Since 2008 about USD 168 million has been invested in the heat generation facilities and the transmission grid, of the Massachusetts DH grid [102].

• • •

4.8.2. Policies and outlook The fact that CHP-DH facilities are storm proof, witnessed during hurricane Sandy, state interest in their development has increased. However, since the fact that the US is a large country, area wise, with most of the population distributed unevenly, this makes it economically unfeasible to implement a large-scale DH grid. Since industries, are the backbone of the American economy, waste heat utilization has an immense potential, especially to serve commercial buildings and urban centres. With the famous executive order by the American President, a target of installing 40 GW more of CHPs by 2020 is initiated. Thirty-four states in the USA have financial incentives, to develop CHP schemes with additional benefits given to sustainable schemes. Several tax-exempt regulations and soft term loans are also provided to these developers. With a huge potential of biomass, solar thermal energy and gas reserves the expansion of sustainable DHC grids is inevitable, especially witnessed by the growth rates in the last few decades. The US has only a general policy towards renewables which in general promotes DHC. More government involvement in introducing heat tariffs, specific DH policies and population awareness, are to be pressed upon in the future. High investment costs and lower pay back rates are driving away investors and to make matters worse local municipalizes have limited access to finances and autonomy, which should change.

grids to provide energy security, competitiveness and cost effective solutions; Automatically managing electrical and heat appliances in buildings for better demand side management techniques; Completely automated and transparent meters with real time figures available to users; and Possible linking up of grids at national levels and interaction with spot markets.

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5. Discussion A summary of the advantages of DH over conventional heating systems as presented in Section 2, are repetitive in the Table 10: A gist of the DH networks in different parts of the world as presented in Section 4, are summarized in Table 11: The major components of a DH grid have evolved into a comprehensive sustainable heating solution over the decades, with past reflections in these trendsetting nations. While on the one hand sustainable generation sources are present in Scandinavia, thorough transmission grids are present in Eastern Europe while innovative distribution topologies are making way in China. At the same time, thermal energy storage is making way to interlink the three components, with innovative research in central Europe. 6. Conclusions With advancements in renewable heating technologies and proactive research, to minimize carbon emissions in the heating sector, DH is evolving to become an essential ingredient in future energy systems. Especially with world leaders in Scandinavia, these research goals are being transformed into practical outputs, for the world to follow. The developments of modern DH schemes along with co-relating these to unique grids worldwide was presented in this paper, with an analysis on their evolutionary policies and future outlooks. An overview of social and economic implications of DH was also put forward. The concept of fourth generation district grids may take lead and transform the current infrastructure to a simpler and low-cost one. Nevertheless, it is expected that like electricity grids we will have smart thermal grids. Some basic features of this anticipative future concept are as follows: 437

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