Energy and CO2 saving potential of district heating system

This paper presents the energy and CO2 saving potential of existing district heating energy system. Analysed system fully rely on fuel oil, with significant energy losses, increased fuel consumption and CO2 emission resulting from outdated and oversized system and fuel with high greenhouse emission factor. Heat generation and thermal energy distribution systems efficiency are assessed, showing that overall system efficiency is 48.5%. System environmental impact is shown via absolute CO2 and specific CO2 emission per heated surface area and useful energy. The study proposes retrofit measures to improve system efficiency, reduce fuel consumption, introduce low-emission fuels, and lower the system’s environmental impact. The study finds that the implementation of these measures could reduce system energy consumption by 42.7%, absolute CO2 emissions by 52%, and specific CO2 indicators as well, highlighting the potential for reducing the environmental impact of district heating systems while meeting users energy needs.


Introduction
Climate change will have a significant and long-lasting negative impact on the environment.Reducing greenhouse gas (GHG) emissions, particularly CO2 emissions, is crucial to preventing the rise in global temperatures.By transitioning from fossil fuels to low-carbon energy sources and renewable energy to achieve net-zero emissions, GHG emissions can be reduced.In response to the Paris Agreement's objectives, numerous governments and business leaders have pledged to reduce carbon emissions, making decarbonisation a global priority critical to limiting global warming.Achieving climate targets for 2030.and 2050.and meeting essential energy needs requires addressing the challenge of decarbonising heating and cooling systems.As cities are contributors to worldwide CO2 emissions and are predicted to expand in the coming years, providing low-emission energy sources to densely populated regions is imperative.This will enable them to meet their energy demands sustainably [1] [2].About 3.5% of the world's CO2 emissions in 2021.are caused by the generating of district heat.To comply with the Net Zero Scenario, CO2 emissions of district heat production must decrease by 20% by 2030.compared to 2021 [3].District heating networks have significant potential for integrating low-carbon energy sources into the heating mix, but globally they have yet to be fully decarbonized, despite being a well-established technology.In central and eastern Europe, many urban district heating systems have outdated technology and issues such as low heat production efficiency, high distribution network losses, and heavy reliance on fossil fuels.The modernisation of these systems to reduce their environmental impact and operational costs is challenging due to poor maintenance [4].Effects of the measures suggested by two strategic documents, regarding the development of the Belgrade district heating system until 2030., are shown in [5], along with some difficulties in their implementation.In order to better understand the structure of the fuel mix for district heating systems and the introduction 1239 (2023) 012019 IOP Publishing doi:10.1088/1755-1315/1239/1/012019 2 of renewable energy sources, four indicators were selected to show how strategic documents affect energy security and GHG emissions.Retrofitting of district heating systems, covering production, distribution, and consumption stages is analyzed in [6].It uses validated district planning tools to calculate and evaluate the district heating system components.The study shows improvements in efficiency of fuel use and indirect reduction in CO2 emissions by retrofitting production units.Nine efficiency and balance indicators are used for quantitative evaluation, and six simulation scenarios are analyzed.The last scenario shows a 40% reduction in CO2 emissions and a 47% share of biomass in fuel mix.Reviews of strategies for reducing the supply temperature in existing district heating networks to meet decarbonization targets is analyzed in [7].The review includes previous studies and experiences, as well as constraints and actions to overcome them.The analysis suggests that temperature reductions can be achieved without major infrastructural modifications.
Analysis presented in this study aims to investigate the positive environmental effects resulting from energy retrofit of district heating system located in south-eastern Europe.The developed retrofit scenario includes the retrofit of the heat generation system, thermal energy distribution system, and the usage of fuel with a low CO2 emission factor.In this study feasible solutions for lowering energy use and CO2 emissions are provided, and transformation of the current district heating system into more sustainable and environmentally friendly.Results provide valuable information that can support policymakers, energy experts, and researchers, in adopting and optimizing existing district heating systems.It is an essential and timely resource in the global dialogue on climate change.
The paper is structured as follows: the introduction is followed by the presentation of the research methodology in Section 2. Section 3 covers the characteristics of the current system configuration, including the heat generation, distribution system, and thermal energy consumers.Section 4 evaluates the energy and environmental characteristics of the existing system, with particular emphasis on energy losses, overall system efficiency, and environmental indicators.Section 5 discusses the evaluation of energy retrofit measures.Section 6 presents the environmental indicators for the current state and after implementing the retrofit measures.Finally, concluding remarks are given in Section 7.

Methods
The objective of this study is to present the potential for energy savings and reduction of environmental impact that could be achieved by applying energy saving measures to the existing district heating system.The system encompasses a heat generation system, a boiler room with steam boilers and thermal energy distribution system (primary and secondary), with function to distribute thermal energy for heating to residential and industrial consumers.System relies exclusively on fuel oil, which contributes to high CO2 emissions.The considered system is a remnant of an industrial setup that was primarily used to supply steam to industrial consumers, while heating for the residential sector was a secondary function.Currently, the system's sole purpose is to provide thermal energy to residential and industrial consumers, with considerably reduced energy demands compared to designed state and no requirement for steam supply.
The study assesses the efficiency of the entire system and its components by analysing annual fuel consumption, heat delivered to residential and industrial consumers, and characteristics and parameters of installed components.System parameters are assessed using the measurements and expert analysis.Heat meters are installed in heating substations, located at the secondary pipeline of residential and industrial consumers.
Energy supplied from fuel,  del is calculated as an average of the energy supplied from fuel oil in three consecutive years.Annual energy supplied is calculated as: where: The boiler room contains two operational steam boilers, one is used as the primary boiler and the other for covering peak performance, with outdated and oversized system components installed in boiler room.Most of the equipment is missing thermal insulation, resulting in losses attributed to external cooling.The primary and secondary distribution pipelines experience significant energy and water losses due to obsolescence and a lack of maintenance.Overall energy losses are calculated using the annual fuel consumption data and heat delivered to the consumers, where environmental impact of the system is shown by absolute and specific CO2 emission.
Absolute value of CO2 emission,  CO2,an is calculated as an average of the CO2 emission in the three consecutive years.Annual CO2 emission is calculated as: where:  CO2,an , is annual CO2 emission, kg/ann.,  f , is annual fuel consumption, kg/ann.,   , is CO2 coefficient per unit of fuel (3.06 kg/kg of fuel oil, and 2.065 kg/m 3 of natural gas) [8].
After examining characteristics of the current system, a set of retrofit measures has been analyzed assuming that the supplied useful energy to consumers will not change.The retrofit scenario involves upgrade of the existing heat generation system and replacement of steam boilers with new fully automated ones.It also includes reconstruction of the pipeline to eliminate water leakage and reduce energy losses, as well as replacement of fuel oil with natural gas, which has a lower CO2 emission factor.The implementation of the retrofit scenario resulted in a decrease of energy consumption and improvement in selected environmental indicators, as demonstrated in this study.
Environmental indicators, selected for the analysis, are absolute value of CO2 emission and specific CO2 emission per heated surface area and useful energy.Specific CO2 emission per heated surface area of residential consumers is calculated as: where:  CO2,A , is specific CO2 emission per heated surface area, kg/m 2 ,  CO2,resid , is annual CO2 emission related to the residential consumers, kg/ann.,  n,heat , is heated surface area of residential consumers of 47.641 m 2 .Specific CO2 emission per useful energy is calculated as: where:  CO2,Ei , is specific CO2 emission per useful energy delivered to the residential or industrial consumers, kg/kWh.,  CO2,i , is annual CO2 emission related to the residential or industrial consumers, kg/ann.,  useful,i , is useful energy delivered to the residential or industrial consumers, kWh.

System Configuration
Analyzed district heating system is located in Sarajevo (Bosnia and Herzegovina), with continental climate and heating degree days (HDDs) 2.968 [9].The system encompasses heat generation system (boiler room) and thermal energy distribution system (pipeline system), with main function to supply thermal energy to industrial and residential consumers.The layout of the analysed system is shown in figure 1. with noted industrial and residential consumers, and thermal energy distribution system connecting the boiler room and consumers.

Heat Generation System
There are a total of three boilers installed in the boiler room, with combined installed capacity of 85 MW.One of these boilers is outdated and out of function.The two operational steam boilers (figure 2.) have a combined installed power of 23 MW.The boiler manufactured in 1980, has a power output of 6.5 MW and steam production capacity of 10 t/hour, and the second boiler, manufactured in 1974., has a power output of 16.5 MW and steam production capacity of 24 t/hour.Working pressure of both boilers is 12 bar.Fuel oil is used as the fuel, stored in a tank located at the vicinity of the boiler room.The components of the boiler room include boilers, heat exchangers, a condensate recovery tank, a boiler feedwater storage unit, circulating pumps, and other auxiliary and safety equipment.Steam boilers are automatically operated, and the system experiences many operating problems and failures.The steam produced in the boilers is transferred to three steam/water heat exchangers with power of 10 MW each (figure 3.), where it is used to heat the water supplied to thermal energy distribution system.The subcooled condensate is distributed to the condensate tank and the boiler feedwater storage unit by pumps.The components of the system are oversized because the condensate tank and boiler feedwater storage were designed to accommodate three boilers, however only two are currently in operation.

Thermal energy distribution system
The delivery of thermal energy to the consumers is done through a pipeline system consisting of two branches.One of the pipeline branches distributes thermal energy to an industrial plant, while the other branch serves residential consumers.The pipeline for industrial consumers has a nominal outer diameter of DN 300, it is completely above ground, in good condition, and has no detectable failures.Total length of initial and return pipelines of distribution system is 1.482 m.Pipeline is made of steel pipes insulated with mineral wool and covered with aluminium sheet.The supply and return pipelines for residential consumers have a total length of 10.628 m.The 420 m of the pipeline, connected to the boiler room, is above ground and is made of steel pipes with a nominal outer diameter of DN 300.These pipes are insulated with mineral wool and covered with aluminium sheet.The pipeline then shifts to an underground concrete channels.Steel pipe network is prone to leaking and covered with worn-out insulation.This poses a major concern, especially because a significant portion of the pipeline is underground, making it challenging to fix leaks.Furthermore, there is a secondary network that connects individual heating substations to heated objects.This secondary network is significantly shorter than the primary network, and the total length of all routes in the secondary distribution system does not exceed 1.000 m.
The design conditions of the distribution system primary circuit are 130/70 ℃ temperature regime, and a secondary circuit 90/70 ℃.However, during analysis of the system, it was found that the actual operating conditions differ significantly, with the temperature regime of the primary hot water circuit is 85/75 ℃ and that of the secondary circuit is 70/60 ℃.

Thermal energy consumers
Residential consumers encompass public buildings (schools, office buildings etc.), and collective houses, with total heated surface area of 47.641 m 2 .Total number of heating substations is 14, with installed power of 10,6 MW.Using the delivered energy data for heating and heated surface area it can be shown that specific final energy for residential consumers vary from 45 kWh/m 2 ann. to 191 kWh/m 2 ann., depending on building construction period and characteristics of building envelope.Average specific final energy for heating of all residential consumers is 66 kWh/m 2 ann.Installed power of industrial consumers is approximately 10 MW.

Energy characteristics of system
Energy supplied by fuel oil, for the period of three years is considered in the analysis.In average, energy supplied by fuel is 16.878 MWh/ann.Boilers efficiency is measured by indirect method, through heat losses in flue gasses and energy losses via boilers casing.It is shown that instantaneous boilers efficiency is approximately 81%.Other energy losses are present in the boiler room, since thermal insulation of equipment is missing, equipment is oversized and outdated, automatic control of operating parameters is missing, resulting in overall energy losses in heat generation system to be around 24,8% of energy supplied, as shown in figure 5.
Absolute values of energy losses of the thermal energy distribution system higher than losses in boiler room as shown in figure 5.This is a result of outdated system, with significant energy losses.Specific energy losses reaching up to 88 W/m for some pipeline branches, which is significantly higher than specific losses of modern district heating systems [10].Energy supplied to residential consumers is calculated as an average of measured value at the heat meters, and it is shown that 3.153 MWh is supplied annually.Energy supplied to the industrial consumers is in average 5.038 MWh/ann.Considering energy supplied via fuel, and energy supplied to the final consumers, it can be concluded that overall system efficiency is 48.5%, imposing the excess system costs and negative environmental impact.
Maintaining a stable supply of thermal energy to consumers is a challenging task due to the presence of outdated and excessively large equipment, as well as various system failures.Additionally, the system do not meet the design parameters regarding the supply temperature, and a significant number of residential users are opting to disconnect from the system.Currently, only 6 MW of the installed power capacity of 10.6 MW is being actively utilized by the district system residential consumers, and the same situation is prevalent among industrial consumers.Therefore, it is imperative to implement energy-saving measures in both the heat generation and thermal energy distribution systems.

System Retrofit
Implementing energy retrofit measures, the effectiveness of a system can be enhanced through the replacement or modification of existing components, leading to improved performance, efficiency, and functionality.The suggested set of retrofit measures include replacing the current heat generation system and steam boilers with new, completely automated boilers.Considering that the existing industrial and residential users no longer require steam, it is sugested to substitute the steam boilers with high efficient hot water boilers.Two industrial hot water boilers, with combined power of 20 MW, are planned for installation to meet the current need for energy, and to meet any system upgrade in the future.The proposed boilers are designed with thermal insulation to minimize casing cooling losses and incorporate advanced technology for low-emission combustion.By installing new boilers, equiped with automatic regulators, the entire boiler room can be fully automated and monitored centrally or remotely.Additionally, other components in the boiler room will be modernized and replaced with properly designed equipment to further reduce energy losses.As a result of these measures, energy will be utilized more efficiently, reducing boiler room losses from 24.8% to 8%.
The retrofit strategy involves replacing the current distribution pipeline with environmentally and economically favorable pre-insulated fused transport steel pipes, suitable for district heating and cooling systems.The insulation of these pipes is composed of polyurethane (PUR) foam, which provides excellent insulation quality with a low thermal conductivity coefficient of λ=0.027W/mK.These pipes offer several advantages over the existing pipelines, including decreased overall heat losses, the incorporation of leak detection systems, straightforward and convenient installation procedures, guaranteed quality and safety, and a long lifespan.Implementation of this measure should result in reduction of specific heat losses from 88 W/m to 25.84 W/m, and significantly decrease losses from 35.5% to 8%.
For the purpose of this analysis it is assumed that energy supplied to consumers (useful energy) will remain unchanged from its current state, as can be seen in figure 6.If any energy saving measures are implemented on the demand side, the current system can be expanded to include additional consumers.As it is shown in figure 6. system annual energy consumption after retrofit is reduced by 42.6%, from 16.878,5 MWh to 9.677 MWh.Furthermore, the replacement of fuel oil with natural gas should reduce the current fuel oil CO2 emission factor from 3.06 kg/kg to 2.06 kg/m 3 , that is CO2 emission factor of natural gas.Transition from the current fuel oil to natural gas should result in significant reduction of CO2 emissions.6. illustrates the absolute values of CO2 at the current state and after the retrofit, which is a result of reduction of energy consumption and lower CO2 emission factor of natural gas in comparison to fuel oil.It is shown that absolute CO2 emission is reduced by 52%, from 4.718 t/ann.to 2.122 t/ann.

Environmental indicators
As shown in the previous section, the environmental impact of the system is assessed by absolute CO2 emission.Table 1.presents the absolute CO2 emission separately for two categories of consumers: residential and industrial.The table also shows the specific CO2 emission per net heating surface and per delivered useful energy, for the residential consumers, and per delivered useful energy for industrial consumers.Due to the fact that the distribution pipelines for those two categories are of different lengths, the two categories show different energy losses at the pipelines.The results indicate that the current environmental indicators have much higher values than those of district heating systems in the region [5] and in Europe [11].System energy retrofit has positive impact on the environmental indicators, with resulting values much lower than the current state for both residential and industrial consumers.Therefore, introducing energy retrofit measures not only leads to significant energy savings, but also significantly reduces emissions, which is very favorable from an environmental perspective.

Conclusion
The energy retrofit of an existing district heating system has been examined in detail in this study, with a focus on improving the current heat generation and thermal energy distribution system, as well as exploring the usage of fuel with more favorable CO2 emission coefficient compared to currently used fuel.
Study results demonstrate that upgrading to more efficient boilers and retrofitting distribution pipelines could reduce energy consumption by 42.6%, leading to a corresponding decrease in CO2 emissions.Furthermore, replacing fuel oil with natural gas can also help in reduction of CO2 emissions.It is found that implementing energy retrofit measures could result in a reduction of absolute CO2 emissions by 52%.
Various environmental indicators are calculated, such as total CO2 emissions, specific CO2 emissions per heated surface area of residential consumers, and per useful energy supplied to the residential and industrial consumers.Presented analysis shows that implementing system energy retrofit has a positive impact on these environmental indicators, resulting in significantly lower values compared to the current state, for both residential and industrial consumers.
The full effect of the analyzed measures could be obtained by simultaneous implementations of all measures.Installed capacity of system allows future increase in number of residential units connected to the system, with expectation that specific environmental indicators will remain the same.
Although, numerous enhancement of district heating systems have been implemented in Europe [1][5] [6], studying pilot projects and case studies outside of Europe can be very beneficial [12] [13].Results of presented study emphasizes the importance of pursuing energy retrofitting as a key strategy for energy efficiency improvement and reduction of district heating systems carbon emissions.The discussion broadens our comprehension of the applicability and advantages of district heating system energy retrofit in various geographic contexts by examining international examples, and aims to encourage and direct international audiences to consider this sustainable solution by highlighting the significance of district heating system energy retrofit in a global context.System retrofitting extends beyond district heating systems to include building retrofits [14], industrial process improvements [15], transportation upgrades [16] and energy grid modernization [17].Examining pilot areas in these sectors offers valuable insights into the challenges, benefits, and best practices of retrofitting, enabling policymakers to apply effective strategies and accelerate the global transition to sustainable practices.

4 Figure 1 .
Figure 1.Map and layout of the district heating system.

Figure 2 .
Figure 2. Steam boilers installed in boiler room.

Figure 3 .
Figure 3. Heat exchanger installed in boiler room.

Figure 4 .
Figure 4. Residential consumers of thermal energy.

Figure 6 .
Figure 6.Energy consumption in the current state and after retrofit.

Figure
Figure 6.illustrates the absolute values of CO2 at the current state and after the retrofit, which is a result of reduction of energy consumption and lower CO2 emission factor of natural gas in comparison to fuel oil.It is shown that absolute CO2 emission is reduced by 52%, from 4.718 t/ann.to 2.122 t/ann.

Table 1 .
Environmental impact in the current state and after retrofit.