The Paris Agreement has turned climate change mitigation and energy decarbonization into global priorities. A number of studies have concluded that a reduction of at least 75% of the energy-related emissions is needed by 2050 to limit global temperature increase to below 2 °C [1
]. Although unprecedent progress has been achieved in the power sector [4
] (with thorough reviews in [7
]), significant efforts are still required to decarbonize the transport and heating sectors [2
]. The European Union (EU) long-term vision for a prosperous, modern, and climate-neutral economy by 2050 stresses the importance of the heating and cooling sectors in achieving significant emissions reductions. Heating and cooling sectors have become priorities for the EU, as they account for 40% of the total final energy demand in Europe and 75% is currently being supplied by fossil fuels [10
]. Apart from being fossil-fuel based, the current energy system is characterized by low efficiencies, a significant amount of waste heat, and a large decoupling between heat and power segments [10
Decarbonizing the heating sector requires a shift in the current paradigm toward a future energy system based on cost-effective multi-sectorial coupling, deployment of efficient technologies, and exploitation of low-carbon energy sources. This new paradigm has been addressed in various studies with different approaches, scopes, and levels of maturity. These studies have proposed various measures to achieve significant emissions reductions, including an increased use of district heating (DH) and combined heat and power generation (CHP) [10
]; a higher penetration of renewables [10
] viz. biomass [16
] and geothermal energies [17
]; increasing use of industrial heat [18
] and unconventional heat sources [19
]; an increased coupling of power and heating sectors [11
]; heat storage [18
] and demand-side management [20
]; and decreasing operating temperatures of the district heating networks [21
], among others.
In Germany, the situation is not much different than in the rest of the EU. Germany struggles to meet its near-term emissions reduction targets in lagging sectors, like heating or transport [24
], and to identify energy transition pathways beyond power generation. In Germany, fossil fuels represent the highest share (75%) of the fuel mix in the heating sector, while the remaining share relies on low-carbon technologies, such as district heating (10%), renewables (10%), and electric heating (5%) [10
]. The prevalence of fossil fuels combined with the large demand has turned Germany into the largest producer of heating-related CO2
emissions in the EU (120 Mio tons CO2
in 2015) [10
]. Various studies have addressed the decarbonization of the heat supply in Germany at country [26
] and municipality [32
] scales. However, little research has focused on decarbonizing large district heating networks in Germany, even though their potential to cost-effectively reduce emissions is significantly larger than in decentralized heating systems [10
In this context, a relevant city is Berlin, not only because it hosts the largest district heating in Germany (2000 km, 10.7 TWh per year) and the third largest in the EU-27 [33
], but also because of its ambitious climate goals. Berlin has set the goal of becoming climate-neutral by 2050 at the latest (Berlin Energy Turnaround Act, EWG Bln. [34
]), which means reducing its CO2
emissions by 95% compared to 1990. With its climate protection goal, Berlin is much more ambitious than the German government, which has only set a reduction range of 80% to 95% by 2050 [35
]. Early on, Berlin recognized that the heat supply is a major contributor to the city’s high CO2
emissions. Heating accounts for 50% of the total final energy demand in the city (66 TWh per year), which is 90% supplied by fossil fuels [36
]. Electricity and renewables supply the remaining 10%. While district heating contributes to one-third of the heating demand (10.7 TWh per year), it is mainly generated from coal (45%) and natural gas (45%) [36
]. Three coal-based CHP units are operated in the city, which emitted 3 Mio tons CO2
in 2017, representing 18% of Berlin’s total emissions [36
]. Therefore, an important field of action is the sustainable and climate-neutral heat supply in the future in Berlin. An essential key component of accomplishing this is the future design of the district heating systems [37
]. Phasing out coal for supplying district heating is one of the most important milestones on Berlin’s path to a decarbonized future. Phasing out of coal already started in 2017, firstly, when the lignite-fired steam generators in Klingenberg were decommissioned and, secondly, when Berlin became the first German state to legally define the coal phase-out by 2030 in the Berlin Energy Turnaround Act (2017, § 15 para. 1 EWG Bln. [34
]). It was not until July 2020 that the German government decided to phase out coal by 2038.
Motivated by the ambitious goal to reduce 95% emissions by 2050, we address the challenge of decarbonizing the Berlin’s district heating network from the combined perspective of the state administration and the system operator. Berlin’s Senate Administration for the Environment, Transport and Climate Protection (SenUVK), together with Vattenfall and B E T, conducted a two-year feasibility study on tangible and climate-friendly transformation paths to replace coal-fired power units in Berlin [39
]. Low-carbon supply options for substituting coal-based heat have been analyzed in detail with regard to their technical and economic feasibility. A monitoring group consisting of representatives of the Berlin parliament, NGOs, local, business, and scientific communities was convened to support, challenge, and monitor the study. This paper presents the methodology employed in the abovementioned study [39
] in a more extensive and detailed way and highlights key findings. The employed methodology combines three key elements: (1) scenario analysis including high-fidelity models of the European power market and the heat demand in Berlin, (2) evaluation of energy potential from low-carbon alternative sources, and (3) a techno-economic portfolio optimization. This combination is advantageous compared to existing studies, which focused only on some of these components. Previous studies typically combined methods for estimating the heat demand and supply, but did not always include sectoral integration or the potential from alternative sources. Methods for estimating the heat demand include those based on historical data, predictive time-series methods, and simulation-based models [40
]. Historical data are preferred but not always available, requiring the use of predictive or simulation methods. Here, we used a high-fidelity simulation model of the entire stock of buildings in Berlin that leverages historical data. Methods used for estimating the heat supply include integrated assessment modeling (IAM), dispatch optimization, merit order allocation, and process simulation. IAM and dispatch optimization offer a wide sectoral and technology coverage at the expense of higher computational intensity. Merit order allocation and process simulation offer faster responses but limited sectoral coverage. Here, we combined a state-of-the-art heat dispatch optimization model with a high-fidelity model of the European power market to consider the integration of heat and power sectors in detail. Additionally, we performed a detailed analysis of the availability of low-carbon alternative heat sources that could replace coal.
We aimed to provide guidelines for similar studies in other cities or regions with similar characteristics or seeking comparable decarbonization goals. This paper is structured as follows: Section 2
discusses the characteristics of the district heating network in Berlin. Section 3
explains the methodology used in this investigation. Section 4
explains the scenario analysis viz. how scenarios describing different climate policy goals are defined; it also describes models of the European power market and the heat demand in Berlin. Section 5
provides an evaluation of the energy potential from different alternative sources to replace coal units and defines transformation paths. Section 6
discusses the heat supply model for evaluating the optimal operation of the different transformation paths (feeding the district heating network) and of decentralized solutions. This section also describes how the dispatch, costs, and emissions are evaluated. Finally, Section 7
presents the most significant results of the investigation followed by discussion in Section 8
and main conclusions in Section 9
In this paper, a methodology combining high-fidelity models of the heat demand and supply, the power market, and the energy potential from alternative resources is proposed, which offers a suitable method of analyzing and evaluating possible energy transition pathways for complex district heating systems. This methodology is envisioned to be sufficiently broad to be employed for other systems or countries.
A scenario frame was developed to comprehensively and transparently consider uncertainties regarding future heat and energy market developments. Then, the technical energy potential for heat generation options in SA1 in Berlin was evaluated, considering technical, environmental, and economic criteria. This allowed a clear estimate of the feasibility and the strategic fit of these options for particular sites and in combination with the rest of the portfolio. Three scenarios with specific transformation paths based on combinations of feasible heat generation options were considered. To evaluate the cost-effectiveness of these transformation paths, various models for dispatch simulation, portfolio optimization, and decentralized supply were created. These models provided an estimation of the power and heat portfolio dispatch with an hourly resolution (considering the hydraulic restrictions of the district heating grid) and the associated CO2 emissions.
Applying this methodology to the example of SA1 in the district heating network in Berlin led to various conclusions. The results showed that a coal phase-out by 2030 at the latest is feasible without any discontinuities in the provision of heat or heating services. While low-carbon sources (including biomass, geothermal energy, municipal solid waste, sewage water heat, and excess heat from industrial and commercial sectors) could partially substitute coal-based heat, they would not be sufficient to replace it completely. Thus, a gas-based (including natural gas, renewable synthetic gas, hydrogen) combined heat and power (CHP) plant linked with a power-to-heat plant would be required to fill the gap between future heat capacity requirements and the potential provided by the renewable heat sources mentioned above. This transformation could reduce CO2 emissions by 2.15 million tons in 2030, which is equivalent to 13% of the emissions of the State of Berlin. District heating offers carbon dioxide emissions per unit that are equal to or even lower than those of the decentralized heat supply options.
The results showed that in the longer term (after 2030), the complete decarbonization of the district heating network is possible, but it is dependent, firstly, on the further decarbonization of the electricity sector and, secondly, on the availability of a national or EU-wide infrastructure transporting hydrogen or renewable gas. Regardless, the proposed generation portfolio for the different scenarios was designed for enabling complete decarbonization by 2050.
From an economic perspective, district heating remains the most cost-effective option compared to decentralized heat supply options, even though costs could rise by a value ranging between 0.7 and 1.7 cents/kWh by 2030, i.e., during the period of gradual substitution of coal-based heat. This indicates that low-carbon district heating networks supplying heat to large cities are feasible and cost-effective, although largely dependent on prices (fuels, electricity, CO2) and the regulatory framework.