High-Efficiency Cogeneration: A Viable Solution for the Decarbonization of Cities with District Heating Systems

Abstract

In a global context marked by increasingly evident climate change and an urgent need to reduce carbon emissions, efficient and environmentally friendly energy solutions are no longer just an option, but a necessity. Decarbonizing cities is an essential process for combating climate change and creating a sustainable urban environment. This article provides an analysis of the decarbonization possibilities of the building heating sector in the case of cities with district heating systems. A case study referring to the district heating system of Suceava city, Romania, is provided. The results of this study show a significant reduction in carbon emissions per unit of thermal energy delivered (95.97%) from the district heating system after 2015 because of the change in technology and primary energy source (cogeneration and biomass). Also, a comparative analysis is provided: district heating vs. individual heating in terms of carbon dioxide (CO₂) emissions for the same amount of heat supplied to end consumers in 2023. The comparative analysis highlights a difference in CO₂ emission of 81.66% (0.220 kg CO₂/kWh for individual heating and 0.040 kg CO₂/kWh for district heating). The implications of high-efficiency cogeneration in the decarbonization of the building heating sector are analyzed and highlighted.

1. Introduction

Energy is the foundation of the progress of our civilization, being essential for the production and consumption of goods and services, from household appliances to transportation and industrial processes. The intensive use of fossil fuels to produce energy has a significant impact on the environment, generating polluting emissions at an alarming rate. Although these fuels have advantages such as high energy output and ease of production, their negative impact on the environment is becoming increasingly evident.

Globally, urban areas consume over 65% of the world’s energy, generating over 70% of carbon dioxide (CO₂) emissions. Therefore, it is important for cities to act as ecosystems of experimentation and innovation to become climate neutral. Elavarasan et al. (2022) analyses effective decarbonization strategies in the context of the European climate-neutral vision [1] and Maya-Drysdale et al. (2020) investigates what strategic practices European cities currently use to promote decarbonization in energy planning [2]. Barriers and pathways to decarbonizing cities are identified in [3].

The European Union’s objective of reaching net zero carbon dioxide emissions by 2050 will require significant changes to energy systems by limiting conversion processes that result in polluting emissions. Climate change and increasingly ambitious goals to reduce global CO₂ emissions are driving the transition from fossil fuels to renewable sources. Efforts are currently being made to promote green and sustainable energy production, with the goal of achieving carbon neutrality, recognizing that an energy transition is essential, despite our current dependence on traditional energy sources. Understanding and quantifying energy efficiency is essential for achieving sustainable practices. Through the meticulous measurement and analysis of various indicators, information can be obtained about the performance of systems, buildings and processes.

Decarbonizing the building heating sector must be included in any serious climate change mitigation effort [4]. Improving the energy performance of buildings is one of the pathways to decarbonize residential buildings and significantly reduce the cost of climate policy [5].

An overview of the current state of district heating and cooling systems in Europe, with information and suggestions on future trends, is presented in [6].

The economic performance of different heating strategies in decarbonizing the thermal sector through coordinated operation with the electrical system is analyzed in [7].

Hansen et al. (2019) compared the levelized cost of heating using district heating and individual heating solutions. The results highlighted district heating as one of the key technologies for achieving climate goals and reducing emissions [8]. Yoon et al. (2015) evaluated and compared the economic value that consumers assign to different types of convenience between individual heating and district heating. Strategies to promote the many external benefits of district heating systems should emphasize not necessarily their lower cost, but their convenience and safety [9].

Vilen et al. (2023) investigated the cost-effective heating of new housing at the city level from a systems perspective in different scenarios. The results indicate that the most efficient heating systems are district heating for apartment blocks and individual heating options for single-family homes with low heat requirements [10].

District heating networks are often presented as a sustainable heating solution, where heat is generated centrally and then distributed to different buildings and homes. A method for temporal and spatial modeling of heat demand for district heating system expansion is presented in [11]. Fallahnejad et al. (2024) evaluated the impact of heat demand and provided information on economic areas, expansion potential and the average costs of heat distribution in a district heating system [12]. The results confirm the need to expand district heating networks to maintain supply levels because of decreasing heat demand.

Jimenez-Navarro et al. (2020) examined the role of cogeneration plants integrated into district heating systems as one of the potential paths to a future decarbonized energy system [13]. Results show that the cogeneration plants increase the efficiency and reduce both the operating costs and the environmental impact of the energy system.

New concepts such as “energy demand management” and “smart district heating networks” are also starting to be used in district heating systems [14,15].

Many district heating systems in Eastern Europe are currently old and inefficient due to a lack of investment in recent decades, leading to an unreliable supply and loss of consumers [16,17]. The need to increase the energy efficiency of district heating systems is obvious. The directions in which research is focused can be summarized in the increasing flexibility and diversification of energy sources used in district heating systems [18,19,20,21].

The integration of renewable energy resources and heat storage in district heating systems is analyzed in [22,23].

Reducing the specific space heating demand in residential buildings, integrating renewable energy sources and using heat pumps in district heating networks are paths that must be applied simultaneously to decarbonize district heating systems [24,25,26,27].

Reducing carbon dioxide emissions and increasing the efficiency of heating systems are research directions with diverse approaches in some previous research works. For example, Liu et al. (2022) analyzed the carbon emissions and energy consumption of natural gas pipelines [28] and Wang et al. (2012) studied a new method for enhancing heat transfer to end users using nanofluids [29].

The integration of both technologies (cogeneration and heat pumps) in district heating systems results in a stronger synchronization of heat and electricity demand for continental climatic conditions [30,31,32]. The joint effect of cogeneration plants and heat storage in district heating systems on efficiency and operating cost is presented in [33]. The defining features of the five generations of district heating (1GDH-5GDH) that differentiate them from previous generations are presented in [34]. The latest generation of district heating (5GDH) facilitates the integration of low-temperature renewable heat sources, but also of a significant number of end users, both consumers and heat suppliers, called prosumers.

A synthesis of previous research studies shows that district heating and cogeneration offer promising prospects in the context of the energy transition and sustainability concerns in the decarbonization of cities. Traditionally, fossil fuels were the main source of heat generation for district heating systems. Currently, district heating systems are gaining ground by expanding the network and diversifying the sources for producing thermal energy and hot water. Solutions exist and differ from city to city, depending on the specifics and energy potential of each region.

While there are many previous papers focused on the topic of cogeneration, high-efficiency cogeneration is less addressed, and the two concepts can often be confused. High-efficiency cogeneration is a more efficient and sustainable variant of cogeneration, with strict energy efficiency requirements. The current methodology for defining high-efficiency cogeneration is relevant and beneficial for promoting efficient and sustainable technologies in district heating systems. However, there is room for improvement, especially in terms of flexibility, adaptability to local conditions, and the integration of new technologies and low-temperature energy sources.

The main objective of this study consists of identifying alternatives for decarbonizing the building heating sector and using biomass as an energy vector to mitigate climate change by replacing fossil fuels.

The implications of high-efficiency cogeneration in the decarbonization of the building heating sector represent the main novelty of the approach proposed in this study. In this sense, the analysis in this study was oriented towards the entire system of the generation, transmission, distribution, supply and use of heat for heating buildings.

The main contributions of this paper can be summarized as follows:

• Application of the proposed calculation methodology in the case of a district heating system with a biomass cogeneration plant;
• A comparative analysis of fulfilling the qualification criteria for energy production in high-efficiency cogeneration in three scenarios;
• A comparison of district heating vs. individual heating in terms of CO₂ emissions for the same amount of heat supplied to end users;
• Identifying solutions to increase energy efficiency and decarbonization for the Suceava district heating system compared to the current situation.

2. Materials and Methods

This section describes the model used to evaluate a district heating system with a cogeneration heat generation source from the point of view of achieving decarbonization targets and energy efficiency (high-efficiency cogeneration). The application of this methodology aims to identify alternatives for the decarbonizing of cities and using biomass as an energy vector to mitigate climate change by replacing fossil fuels.

The key variables tracked were carbon dioxide emissions and the amount of electricity generated in high-efficiency cogeneration. The results obtained were compared with the reference values established in the European directives presented below.

Figure 1 shows milestones in the chronology of European directives on combating climate change (energy efficiency, cogeneration and greenhouse gas emissions).

The decarbonization targets for district heating systems set out in Directive (EU) 2023/1791 are presented in Figure 2 [35]. Thus, Directive (EU) 2023/1791 sets phased thresholds, applicable between 2027 and 2050, for the gradual increase in the share of renewable energy sources and the reduction in greenhouse gas emissions in district heating systems.

The efficiency of cogeneration is defined by the primary energy savings achieved by combined production compared to the separate production of electricity and heat. A primary energy savings of more than 10% justifies the use of the expression “high-efficiency cogeneration”. Also, in the case of low-power and micro-cogeneration units, cogeneration production that ensures primary energy savings can be considered as high-efficiency cogeneration.

Therefore, high-efficiency cogeneration means cogeneration that meets the criteria set out in Annex III of Directive (EU) 2023/1791 (updated Directive 2012/27/EU) on energy efficiency (

Table 1. High-efficiency cogeneration—necessary criteria.

CHP Unit Power	Primary Energy Saving (PES)	Quality Factor (QF)	Total Efficiency (ηgl,CHP)
Pe ≤ 1 MWe	≥0%	≥1.00001	unrestricted
1 < Pe ≤ 25 MWe	≥10%	≥1.11112	unrestricted
Pe > 25 MWe	≥10%	≥1.11112	≥70%

).

The mathematical model used to verify the criteria that define high-efficiency cogeneration is presented below.

The primary energy saving (PES) can be expressed in terms of the quality factor of the cogeneration unit (QF) [36]:

$$PES=\left(1-{\displaystyle \frac{1}{\frac{{\eta }_{e,CHP}}{{p_{loss}·\eta }_{e,Ref}}+\frac{{\eta }_{h,CHP}}{{\eta }_{h,Ref}}}}\right)\times 100 \left(\%\right)$$

(1)

Or

$$PES=\left(1-{\displaystyle \frac{1}{QF}}\right)·100 \left(\%\right)$$

(2)

The quality factor considers alternative options for the separate production of electricity (X) and heat (Y):

$$QF=X·{\eta }_{e,CHP}+Y·{\eta }_{h,CHP}$$

(3)

where X is the coefficient that considers the alternative option for separate electricity generation:

$$X={\displaystyle \frac{1}{{p_{loss}·\eta }_{e,Ref}}}$$

(4)

and Y is the coefficient that considers the alternative option for separate heat generation:

$$Y={\displaystyle \frac{1}{{\eta }_{h,Ref}}}$$

(5)

The meaning of the terms used in Equations (1)–(5) is as follows:

• η_e,Ref is the efficiency reference value for the separate production of electricity (%);
• η_h,Ref is the efficiency reference value for the separate production of heat (%);
• p_loss is the correction factor for avoided losses in the electrical networks (-);
• η_e,CHP is the electrical efficiency in cogeneration (%);
• η_h,CHP is the heat efficiency in cogeneration (%).

The efficiency reference values for the separate production of electricity and heat are differentiated by relevant factors according to Commission Delegated Regulation (EU) 2023/2104 (

Table 2. Efficiency reference values for separate production of electricity (%) [37].

Energy Source	Year of Construction
	Before 2016	2016–2023	From 2024
Hard coal	44.2	44.2	53.0
Lignite	41.8	44.2	53.0
Natural gas	52.5	53.0	53.0
Heavy fuel oil	44.2	44.2	53.0
Biomass	33.0	37.0	37.0

and

Table 3. Efficiency reference values for separate production of heat (%) [37].

Energy Source	Year of Construction
	Before 2016		2016–2023		From 2024
	Hot Water	Steam	Hot Water	Steam	Hot Water	Steam
Hard coal	88.0	83.0	88.0	83.0	92.0	87.0
Lignite	86.0	81.0	86.0	81.0	92.0	87.0
Natural gas	90.0	85.0	92.0	87.0	92.0	87.0
Heavy fuel oil	89.0	84.0	85.0	80.0	92.0	87.0
Biomass	86.0	81.0	86.0	81.0	86.0	81.0

): year of construction, type of fuel, type of heat supplied and technology used.

In the case of using several types of fuel in the cogeneration plant, the reference efficiency values for the separate production of electricity are weighted as follows:

$${\eta }_{e,Ref}=\sum _{j=1}^m\left(b_j·{\eta }_{e,Ref,j}\right)$$

(6)

If the useful heat is delivered in different forms of thermal agent (direct use of exhaust gases/steam/hot water) and several types of fuel are used in the cogeneration plant, the reference efficiency value for separate heat production is weighted as follows:

$${\eta }_{h,Ref,j}=\sum _{k=1}^p\left(q_k·{\eta }_{h,Ref,k}\right)$$

(7)

$${\eta }_{h,Ref}={\displaystyle \frac{\sum _{j=1}^m\left(Q_j·{\eta }_{h,Ref,j}\right)}{\sum _{j=1}^m{Q}_j}}$$

(8)

The meaning of the terms used in Equations (6)–(8) is as follows:

• b_j is the share of fuel consumption of type j in total fuel consumption (-);
• η_e,Ref,j is the efficiency reference value for the separate production of electricity when operating on fuel j (%);
• q_k is the share of the delivered heat agent (direct use of exhaust gases/steam/hot water) (-);
• η_h,Ref,k(j) is the efficiency reference value for the separate production of heat related to the thermal agent used (exhaust gases/steam/hot water) and the fuel type j (%);
• Q_j is the energy consumed from fuel type j (MWh).

For cases where the cogeneration unit does not operate in full cogeneration mode under normal conditions of use, it is necessary to identify the electricity and heat that are not produced in cogeneration mode and distinguish them from the cogeneration production (Figure 3).

Non-CHP electricity means electricity generated by a cogeneration unit in a reporting period, where one of the following situations occurs: the related thermal energy is generated by the cogeneration process or part of the thermal energy generated cannot be considered useful thermal energy (e.g., heat given off to the cooling tower).

Heat demand is the decisive aspect in justifying the efficiency of the cogeneration solution and is the basic element for qualifying electricity in high-efficiency cogeneration. If the quality factor determined by Equation (3) is lower than the minimum value, the amount of electricity that can be considered as being produced in high-efficiency cogeneration is recalculated.

For this purpose, the efficiency values for the combined production of electricity and heat are recalculated to achieve the minimum quality factor (QF_min):

$${\eta }_{h,HEC}={\displaystyle \frac{{QF}_{min}-{X·\eta }_{e,CHP}}{Y}}$$

(9)

$${\eta }_{e,HEC}={\eta }_{e,CHP}$$

(10)

The power-to-heat equivalent ratio C_ech is calculated by the equation:

$$C_{ech}={\displaystyle \frac{{\eta }_{e,HEC}}{{\eta }_{h,HEC}}}$$

(11)

The electricity generated in cogeneration mode (E_CHP):

$$E_{CHP}=H_{supplied}· C_{ech}$$

(12)

The assessment of the potential for high-efficiency cogeneration is carried out in several stages, starting with an understanding of the infrastructure and operating requirements of the cogeneration system.

To compare the energy consumption for heating buildings, the specific heating consumption was used, calculated as follows:

$$C_{SH}={\displaystyle \frac{H_{supplie\mathrm{d}}}{S_{heated}}} \left({\displaystyle \frac{\mathrm{k}\mathrm{W}\mathrm{h}}{m^2·year}}\right)$$

(13)

where

H_supplied is the useful heat supplied to consumers (kWh/year);

S_heated is the useful heated surface of buildings (m²).

Biomass from forestry and related wood-based industries may be considered CO₂-neutral if it complies with the sustainability criteria set out in Article 29 (6) and (7) of Directive (EU) 2018/2001 [38]. Figure 4 shows the decision tree for applying the sustainability and greenhouse gas (GHG) reduction criteria to biomass [39].

The conversion factors used to determine the equivalent CO₂ emissions in this paper are presented in

Table 4. Conversion factors used to determine CO₂-equivalent emissions [40].

Fuel Type	Emission Factor (t CO2/TJ)	Net Calorific Value (TJ/Gg)	Emission Factor (kg CO2/kWh)
Lignite	101.0	11.9	0.364
Hard coal	94.6	28.2	0.341
Heavy fuel oil	73.3	42.3	0.264
Natural gas	56.1	48.0	0.202
Wood/wood waste	0	15.6	0
Other primary solid biomass	0	11.6	0

[40].

Biomass represents an appropriate ecological alternative to fossil fuels if it is exploited in a sustainable manner. Biomass is currently considered a renewable resource that has two major advantages: it stores solar energy through biological processes and transfers carbon dioxide from the atmosphere to the biosphere, so, by definition, energy production from biomass is a neutral process in terms of CO₂ emissions. However, biomass combustion can only be considered neutral in terms of emissions or with a very low conversion factor if it is certified by guarantees of origin that attest to the source of origin with reference to sustainable and long-term exploitation. As a result, local biomass resources can be used more energy efficiently in cogeneration applications. The problems caused by polluting gases resulting from incomplete combustion in small and inefficient plants are thus avoided.

Table 5. Conversion factors used to determine CO₂-equivalent emissions for biomass [41].

Fuel Type	Emission Factor (kg CO2/kWh)
Firewood (without biomass certification/unsustainable source)	0.390
Biomass—firewood	0.019
Biomass—wood waste, sawdust	0.016
Biomass—briquettes/pellets	0.039
Biomass—agricultural waste	0.016
Biogas (from certified biomass)	0.000

presents the conversion factors of equivalent CO₂ emissions depending on the type of biomass and the degree of fulfillment of the sustainability criteria, provided for in Romanian legislation [41].

In Romania, the origin of biomass is confirmed by environmental protection authorities. The ISO 38200 standard [42] helps buyers track biomass batches from different sources, thus helping to avoid the introduction of wood from illegal sources into the supply chain.

Carbon dioxide emissions in this study were calculated using the appropriate conversion factors (Table 4 and Table 5) with the following equation:

$$E_{CO2}=\sum _{j=1}^n\left(E_{p,j}·f_{CO2,j}\right)$$

(14)

where

• E_p,j is the consumption of primary energy (fuel) of type j (kWh/year);
• f_CO2,j is the conversion factor into CO₂-equivalent emissions for primary energy type j (kgCO₂/kWh).

3. Case Study: District Heating System of Suceava City, Romania

This section reports operating data about the cogeneration plant and district heating system serving of Suceava city, as well as historical data about the heat generation sources.

Suceava is a city in northeastern Romania. The district heating system of Suceava city was developed in stages starting in 1965 as the city expanded.

The sources of thermal energy production for the Suceava district heating system, depending on the primary energy source and operating period, are as follows:

• 1964–2015 hydrocarbon thermal plant (withdrawn from operation):
  • 2 industrial steam boilers of 105 t/h (17 bar; 250 °C);
  • 3 hot water boilers of 58.15 MW;
  • 2 hot water boilers of 116.30 MW;
  • 1 peak boiler of 17.45 MW.
• 1987–2013 coal-fired combined heat and power plant (CHP) 2 × 50 MWe (withdrawn from operation):
  • 2 steam boilers of 420 t/h (137 bar; 540 °C) operating on lignite (1987–2000) followed by conversion to hard coal (2000–2013);
  • 2 power units of 50 MWe (condensing steam turbines and sockets type DSL-50-1);
  • 2 basic boilers 2 × 98.86 MW;
  • 3 peak boilers 3 × 46.52 MW.
• 2015–present BIOENERGY cogeneration plant (in operation):
  • Steam turbine cogeneration unit (29.65 MWe; 71.43 MWt) running on biomass;
  • 1 hot water boiler operating on biomass 15 MWt;
  • 3 hot water boilers operating on natural gas 3 × 14.7 MWt.

3.1. A Brief Presentation of the District Heating System and the Cogeneration Plant

The district heating system includes all activities regarding the production, transport, distribution and use of thermal energy to provide the thermal energy necessary for heating and preparing hot water for consumption.

The district heating system of Suceava city includes the following:

• Thermal energy generating plant (Figure 5).
• Transport networks, with a length of approximately 55 km, of which approximately 72% of the route is underground, and the rest is above ground. The transport networks are made of steel pipes with diameters between Dn 800 and Dn 50, insulated with mineral wool mattresses protected with galvanized sheet metal.
• A total of 59 district heating substations with plate heat exchangers for both heating and domestic hot water. All district heating substations are metered.
• The distribution networks, with a length of 455 km, with diameters ranging from DN 15 to DN 300, are laid in thermal channels. Their thermal insulation is made of mineral wool, protected with polyethylene foil or asphalt cardboard, or polyurethane foam insulation.
• End-users: apartments, public institutions and economic agents.

The heat transport and distribution system were developed in stages, starting in 1965, so currently, a large part of the component elements are almost 60 years old. During the period 2007–2015, 16 district heating substations out of a total of 59 were rehabilitated/modernized and some of the district heating pipes were replaced with pre-insulated pipes.

The schematic diagram of the cogeneration plant is shown in Figure 5. The hot water boilers were sized to ensure peak thermal load and also to enable use during periods of unavailability of the cogeneration unit.

The district heating system of Suceava city is considered a large-sized system with over 10,000 consumers. In 2023, the total number of consumers was 15,459: 15,021 apartments (97.17%); 37 public institutions (0.27%); and 401 economic agents (2.59%). The total percentage of disconnections in the period 2000–2006 was about 22%, with peaks recorded in the years 2001–2003, and starting with the years 2005–2006, disconnections decreased by about 80% compared to 2003. This indicates the trust of an important segment of the population in the district heating system.

3.2. Production Data and Heating Energy Needs of Suceava City

The production data for the district heating system of Suceava city in the period 2019–2023 are presented in

Table 6. Production data for district heating system of Suceava city.

Energy Flow	U.M.	2019	2020	2021	2022	2023
Electrical energy generated	MWh	108,932	126,140	222,245	218,612	126,678
Thermal energy generated	MWh	230,360	253,798	284,501	265,963	205,023
Biomass consumption	MWh	549,965	592,680	950,335	1,051,639	635,576
Natural gas consumption	MWh	656.28	0	0	0	0
Thermal energy delivered	MWh	188,913	179,004	194,515	174,269	167,391
Thermal energy sold to final consumers	MWh	112,397	101,407	99,330	98,352	94,726

[43].

Figure 6 shows the evolution of thermal energy sold to consumers in Suceava city connected to the district heating system during the period 2010–2023 [44].

In recent years, biomass has been the main fuel for the district heating system of Suceava city due to the availability of this primary energy resource in the area. Biomass burns in the boilers of the cogeneration plant without combustion support and comes from the primary industrialization of wood (chips, bark and sawdust) and from the collection of residues left after forest cleaning (branches, tree bark, logs etc.).

The disconnection of some consumers from the district heating system but also some partial works to increase the energy performance of buildings at end consumption were the main causes of this trend of reducing the annual quantity of thermal energy sold to end consumers.

Difficulties in supplying biomass to the cogeneration plant in 2015 were the cause of the deviation from the general trend of reducing the quantity of thermal energy supplied.

4. Results and Discussion

The district heating system of Suceava city was developed in stages starting in 1964 because of the development and expansion of the city through the construction of new residential and non-residential buildings. During this period, heat for the district heating system was provided from different technologies and primary energy sources which are presented in Section 3: coal, fuel oil, natural gas and biomass.

Table 7. Qualification of electricity production in high-efficiency cogeneration.

Energy Flow	U.M.	Scenario 1	Scenario 2	Scenario 3
Power, P	MWe	29.65	15.92	15.92
Heat, H	MWt	71.43	20.27	42.56
Fuel, F	MW	126.17	73.48	73.48
Reference electricity efficiency, ηe,Ref	%	33.00	33.00	33.00
Reference heat efficiency, ηh,Ref	%	86.00	86.00	86.00
Correction factor for avoided losses, ploss	-	0.918	0.918	0.918
Alternative option for separate electricity generation, X	-	0.0330	0.0330	0.0330
Alternative option for separate heat generation, Y	-	0.0116	0.0116	0.0116
Electric efficiency in cogeneration, ηe,CHP	%	23.50	19.93	21.66
Heat efficiency in cogeneration, ηh,CHP	%	56.61	32.26	57.93
Total efficiency in cogeneration, ηgl,CHP	%	80.11	52.19	79.59
Quality factor of cogeneration unit, QF	-	1.434	1.033	1.389
Primary energy saving, PES	%	30.26	3.19	27.99

presents the comparative results of the calculation regarding the fulfillment of the qualification criteria for energy production in high-efficiency cogeneration in three scenarios, using Equations (1)–(8):

• Scenario 1: Nominal technical data for the cogeneration unit;
• Scenario 2: Operating data achieved in 2023 (excluding the heat lost to the cooling tower);
• Scenario 3: Operating data obtained in 2023 (including the heat given off to the cooling tower as heat potential available for connecting new consumers).

The results of this study show that in the current scenario (Scenario 2) the qualification criteria for electricity production in high-efficiency cogeneration are not met:

• Primary energy saving: PES = 3.19% < 10%;
• Total efficiency: η_gl,CHP = 52.19% < 70%.

The cogeneration unit operated at partial loads depending on the heat demand in the district heating system but also depending on the electricity demand in the liberalized energy markets: Centralized Market for Electricity Bilateral Contracts (CMBC); Day-Ahead Market (DAM); and Balancing Market (BM). Participation in liberalized energy markets requires adapting the operation of the cogeneration plant according to the revenue potentials of the different markets.

The load on the electrical side of the cogeneration group in 2023 was 54% of the nominal load. The heat lost at the cooling tower significantly influences the qualification of the electricity production in high-efficiency cogeneration. Scenario 3 highlights this aspect compared to Scenario 2:

• Primary energy saving: PES = 27.99% > 10%;
• Total efficiency: η_gl,CHP = 79.59% > 70%.

In scenario 2 (operating data achieved in 2023), because QF < QF_min, using Equations (9)–(12), the quantity of electricity generated in cogeneration was calculated to identify the high-efficiency cogeneration mode.

Table 8. The separation of the quantities of energy generated in cogeneration and non-cogeneration regimes.

Energy Flow	U.M.	2023
Electricity generated in cogeneration mode, ECHP	MWh	102,610.94
Electricity generated in non-cogeneration mode, Enon-CHP	MWh	24,067.06
Heat generated in cogeneration mode, HCHP	MWh	161,295.35
Heat generated in non-cogeneration mode, Hnon-CHP	MWh	43,727.60
Heat lost to the cooling tower, Hcooling tower	MWh	177,424.88
Fuel consumed in cogeneration mode, FCHP	MWh	584,729.48
Fuel consumed in non-cogeneration mode, Fnon-CHP	MWh	50,846.04

shows the separation of the quantities of energy produced in cogeneration and non-cogeneration regimes, highlighting the quantity of electricity generated in high-efficiency cogeneration.

The results obtained show that the electricity generated in high-efficiency cogeneration was 81% of the total electricity generated in 2023. Therefore, considering that the cogeneration unit operated at 54% of its nominal capacity (but even in this case not all the electricity was generated by high-efficiency cogeneration (only 81%)), there is a significant potential for the useful heat available for connecting new consumers to the district heating system.

Carbon dioxide emissions were calculated using Equation (14) in different operating scenarios depending on the heating sources used, with the results obtained being highlighted in Figure 7 and Figure 8. Figure 7 shows the comparative specific CO₂ emissions (kgCO₂/kWh) for thermal energy supplied to consumers connected to the district heating system, calculated as average values for the respective periods depending on the mix of primary energy sources used. In the first stage of the analysis (2010–2014), the disconnection of some consumers from the district heating system and the switch to individual gas heating sources had a significant impact in terms of reducing greenhouse gas emissions compared to providing heat from the district heating system using coal and fuel oil as primary energy sources. Currently, the solution of individual gas heating is no longer a viable solution in terms of gas emissions in cities with high population densities. In the case of Suceava city, a significant reduction in carbon dioxide emissions per unit of energy supplied (95.97%) occurred after 2015 because of the change in technology and primary energy source (high-efficiency cogeneration and biomass). Thus, the coal-fired combined heat and power plant was withdrawn from operation and replaced with a cogeneration plant operating on biomass and natural gas (Figure 5). The thermal energy generation technologies for the Suceava district heating system, depending on the primary energy source and operating period, were presented at the beginning of this section.

Figure 8 shows the comparison of district heating vs. individual heating in terms of CO₂ emissions for the same amount of heat supplied to end consumers in 2023.

The following calculation assumptions were considered:

I.

District heating

Calculation assumptions:

• Amount of heat supplied to final consumers in 2023 from the district heating system: 94,726 MWh;
• All losses from the source to the final consumers were considered: losses on the transmission and distribution networks (43.41%), losses at the producer and self-consumption (18.36%) levels;
• The allocation of fuel consumption (biomass) for heat generation in cogeneration was made according to the “Alternative heat generation method” [45] with a reference efficiency η_h,Ref = 86%.

Result:

• Carbon dioxide emissions: 3814.38 tons CO₂.

II.

Individual heating

Calculation assumptions:

• The same amount of heat supplied to final consumers in 2023 from the district heating system if it had been generated in individual gas heating sources: 94,726 MWh;
• Individual heating source: gas boiler with a reference efficiency η_h,Ref = 92%.
• Result:
• Carbon dioxide emissions: 20,798.61 tons CO₂.

The comparative analysis highlights a reduction in carbon dioxide emissions of 81.66%.

What possibilities/opportunities for increasing energy efficiency and decarbonization are there for the Suceava district heating system compared to the current situation?

A.

Heat use area at the level of end consumers:

• Energy renovation/rehabilitation of apartment buildings. There is a potential to increase the energy efficiency of buildings by reducing specific heating consumption below 100 kWh/m²·year. For comparison, the specific heating consumption in 2023 was 180 kWh/m²·year (calculated using Equation (13)).
• Attracting new customers. Connecting apartments/public institutions/economic agents located in the DH area:

  ○

  number of apartments connected to DH in 2023: 15,021 (44.70%);

  ○

  total number of apartments that could be connected to DH: 33,604.

• Connection of new buildings located in the DH area:

  ○

  meeting NZEB (Nearly Zero Energy Building) requirements for new buildings.

• Heat metering at apartment level (installing smart equipment that optimizes energy flows based on real demand).

B.

Heat transport and distribution area:

• Reducing losses in heat transport and distribution networks by replacing classic thermal pipes with pre-insulated pipes (losses in heat transport and distribution networks in 2023 were 43.41%);
• Rehabilitation of recirculation pipes for hot water consumption (ensuring optimal supply parameters);
• Refurbishment, automation and remote monitoring of district heating substations;
• Real-time monitoring of heating networks to quickly detect faults or heat loss.

C.

Heat generation area:

• Follow-up in operation of the efficiency of heat generation in cogeneration (fulfillment of the criteria for high-efficiency cogeneration);
• Diversification of production sources/modular sizing/use of renewable energy sources (other than biomass);
• Integration of modern technologies for the efficient utilization of primary energy sources with thermal potential at medium and low temperatures (cogeneration with ORC technology and heat pumps);
• Using heat storage to increase the flexibility of the district heating system.

The results obtained in this study show that the decarbonization targets for the district heating system of Suceava city set out in Directive (EU) 2023/1791 (Figure 2) have been met. Greenhouse gas emissions per unit of heat delivered to customers from the district heating system of Suceava city were 0.040 kg CO₂/kWh. If the biomass did not have guarantees of origin, the greenhouse gas emissions criterion would not have been met. On the other hand, from an energy efficiency point of view, the criteria for “high-efficiency cogeneration” are not met. Therefore, solutions for rehabilitation and increasing energy efficiency are necessary.

The current district heating system of Suceava city has the characteristics of third-generation district heating (3GDH). The defining feature of 4GDH and 5GDH systems is to operate heat distribution systems at low temperatures [46]. The need for significantly larger pipe diameters to allow heat delivery temperatures (less than 45 °C) is considered a disadvantage for the fifth-generation district heating (5GDH). From this perspective, the current infrastructure of the district heating system of Suceava city can be an advantage for the transition to the next generations (4GDH and 5GDH). The current infrastructure can be considered oversized (large pipe diameters and high flow rates) for the quantities of heat that are delivered to end users. As a result, the premises are created for reducing the temperature of hot water but also for integrating low-temperature energy sources (for example, heat pumps).

The availability of power installed in the cogeneration unit, but also the continuation of investments in the modernization and rehabilitation of the heating system, create conditions for connecting new consumers who currently use individual gas heating. The expansion of the district heating system to areas with high population densities will have beneficial effects in reducing carbon dioxide emissions compared to the current model, in which a significant number of apartments have individual gas heating sources.

The district heating system of Suceava city has faced numerous problems in the past due to outdated infrastructure, underfinancing or lack of investment, but also dependence on fossil fuels (coal and fuel oil). The efforts to modernize, develop and streamline the district heating system in recent years have, as their main goal, the regaining of the trust of end consumers in this public utility system. The results of this study show a significant reduction in carbon emissions per unit of energy delivered (95.97%) from the district heating system after 2015 because of the change in technology and primary energy source (cogeneration and biomass).

Currently, 44.70% of the apartments in the city of Suceava are connected to the district heating system, the rest of the apartments having individual gas heating sources.

The comparative analysis between individual heating and district heating highlights a difference in carbon dioxide emissions of 81.66% (0.220 kg CO₂/kWh for individual heating and 0.040 kg CO₂/kWh for district heating).

The selection of Suceava city within the EU Mission “Smart and climate-neutral cities—100 climate-neutral cities by 2030” represents an important motivation in increasing the energy efficiency of the district heating system as part of urban development plans in achieving sustainability objectives [47].

5. Conclusions

The diversity of primary energy sources that can be used by cogeneration technologies makes high-efficiency cogeneration a viable option in the decarbonization of cities with district heating systems. In urban areas with high population densities, district heating is the only possible solution that can integrate large amounts of heat from renewable sources into the building heating sector.

High-efficiency cogeneration should be analyzed in the context of the energy transition, not only against conventional systems. The analysis should take into account several factors, including the relevance of the criteria, comparability with separate generation alternatives, flexibility in different energy contexts and adaptability to local conditions, and the integration of new distributed technologies located close to users.

The results obtained in this study show that the decarbonization targets for the district heating system of Suceava city set out in Directive (EU) 2023/1791 are met. Greenhouse gas emissions per unit of heat delivered to customers from the district heating system of Suceava city were 0.040 kg CO₂/kWh. If the biomass did not have guarantees of origin, the greenhouse gas emissions criterion would not have been met. On the other hand, from an energy efficiency point of view, the criteria for “high-efficiency cogeneration” are not met (primary energy savings: PES = 3.19% < 10%; total efficiency: η_gl,CHP = 52.19% < 70%). Therefore, solutions for rehabilitation and increasing energy efficiency are necessary. Also, Scenario 3 highlights the potential of useful heat for connecting new users to the district heating system.

Applying the calculation methodology for high-efficiency cogeneration presupposes a very good knowledge of the technology and the particularities of the operation of the cogeneration plant for the correct identification of the quantities of energy generated in cogeneration and non-cogeneration regimes. Therefore, the results obtained and the conclusions of this study refer to the case study considered. This aspect represents one of the limitations of this study.

Also, the energy and environmental performances of a heating system depend on the involvement of local public authorities in promoting plans aimed at maximizing the use of renewable resources available at the local level, reusing residual heat and developing heat storage solutions. To maximize the impact, these measures should be complemented by ambitious building renovation plans.

Future research directions will be oriented towards the integration of modern technologies for the efficient use of primary energy sources with thermal potential at medium and low temperatures (cogeneration with ORC technology and heat pumps) and the use of heat storage to increase the flexibility of the district heating system.