Decarbonization of a district heating system with a combination of solar heat and bioenergy: A techno-economic case study in the Northern European context
Introduction
Integration of significant amounts of variable renewable energy (VRE) to the power sector and increased need for flexibility have received a lot of attention in the context of energy transition [1,2], while the challenge of cutting emissions in the heating and cooling sector has gained less attention. Heating and cooling in buildings and industry account for 47% of the energy consumption in EU-28 [3,4], and the share of fossil fuels in gross heat production remains at 73% [5]. Emissions reduction potential in heating sector is therefore significant, and has lately been addressed for example through publication of the EU Strategy on Heating and Cooling in 2016 [6]. The strategy promotes renewable energy production and the integration into district heating and cooling (DHC) systems, providing flexibility to the energy system by balancing supply and demand through thermal energy storage (TES) systems. RED II ((EU) 2018/2001) sets renewable energy targets in DHC sector in Member States [7].
District heating (DH) has been identified as one of the key measures for decarbonizing the heating sector, and for enabling exploitation of VRE sources [8]. Renewable energy sources (RES) represent 26.5% of gross heat production in EU-28 (2017), of which solid biomass contributes 72% [5]. The role of biomass is expected to further strengthen in DH sector [9]. Hakkarainen & Hannula [10] concluded in their review that as storable and dispatchable resource bioenergy has potential to support VRE integration at different scales in heating and cooling applications, through power-to-gas concepts and through long-term storage. Biomass can exist as solid, liquid and gaseous forms, and it can be used in power, heating, cooling and transport sectors [11]. Sikkema et al. [12] compares the share of solid biomass in gross energy final consumption in EU Member States and ranks Finland, Sweden and Denmark among the largest biomass consumers.
Bioenergy is predicted to be in the key role to achieve EU's RE targets for 2030 and beyond [13]. As pointed out by Cross et al. [14] in their study about the impact of policy landscape on actual bioenergy development, the combination of DH networks and bioenergy has provided a sharp transition from fossil to renewable fuels in heating sector in Nordic countries. However, there has been signs of growing competition of limited biomass feedstock between different sectors. Jordan et al. [15] concluded in their study on cost-optimum allocation of limited biomass potential in Germany that in the long-term the biomass use shifts from the household sector to high temperature industry applications. As a consequence of the possible competition, the price of biomass may increase [16] and even become one of the most expensive renewable heat source [17], which creates pressure for the biomass-based heat producers [18]. Soltero et al. [19] showed that forest biomass DH facilities in rural areas in Spain can result in 20% decrease in end-use energy costs compared to diesel boilers, but only 1.2% of studied municipalities can cover the biomass demand with local resources. Also the sustainability criteria for biomass use has been widely discussed recently. Harjanne & Korhonen [20] noted that biomass use may be sustainable when used in a local, small-scale manner, but unsustainable if used to power large cities. Quirion-Blais et al. [21] pointed out public acceptance related to the feedstock requirement and the associated logistics as a challenge for biomass use in DH.
District heating networks are inherently local and use local fuels and heat sources in order to supply local heat demand [22]. The current 3rd generation DH technology was developed for fossil fuels, but the emerging 4th generation technology needs to incorporate diversifying renewable and waste heat sources, and operation at low temperature levels [23]. It is commonly agreed in literature, for example by Lizana et al. [24], Vad Mathiesen et al. [11] and Tian et al. [25], that combination of different heat sources, such as biomass, solar energy, waste heat, heat pumps, geothermal and waste incineration coupled with TES are becoming increasingly interesting for the DH networks in the pathways towards 100% renewable DH networks. The operation of the DH system is based on optimizing the unit commitment and load dispatch in an effort to reduce the total heat cost for each hour of production [18]. As the resource base widens along the increasing VRE share also the operation of such a system becomes more complicated and has new requirements [26]. A potential challenge in complex multiple heat source systems is the high initial investment cost [27].
Solar district heating (SDH) systems can be categorized in centralized and distributed systems. Centralized systems include a large-scale ground-mounted solar field, typically coupled with a thermal energy storage system, whereas distributed systems are located on rooftops of single buildings to supply their heat demand [28,29]. Rämä and Mohammadi [29] compared these two options and concluded that a centralized solution outpaces the distributed solutions in terms of performance and costs. Fisch et al. [30] concluded based on their review that in large-scale solar applications, solar heat cost can be cut at least in third compared to use in domestic applications. In Denmark, several investments in large-scale SDH installations have already taken place and sound growth has been seen in several other countries, such as Austria, Germany and Sweden [31]. By the end of 2019, about 400 large-scale solar thermal systems (>350 kWth, 500 m2) were reported to be in operation in DH applications [32].
The combination of different RES in DH networks could reduce pressure from limited biomass resource and release it for other sectors. Vad Mathiesen et al. [11] studied different scenarios for 100% renewable energy system in Denmark. The results show that expansion of DH can reduce pressure on the limited biomass resource, and is important especially in the case of large amounts of VRE. Hansen & Vad Mathiesen [28] showed that solar thermal energy could reduce biomass consumption by 1–3% in Germany, Austria, Italy and Denmark. However, the variable renewable heat integration sets new requirements for bioenergy utilization. In small DH systems in towns and villages, heat-only boilers (HOB), often combined with flue gas condensation, are typically the main or only heat source [33] and operated as base load producer. Tereshchenho & Nord [18] concluded in their Norwegian case study that biomass HOB should be used as base load producer, as the sensitivity of levelized cost of energy (LCoE) on fuel price variation increases if the plant is operated at intermediate load. However, Szarka et al. [17] shows that the role of bioenergy shifts from being a base load generator towards being a flexible stabilizing energy source both in heat and power sectors in Germany, when aiming at 95% GHG reduction. Consequently, there will be an increased demand for more flexible conversion technologies due to a growing need for energy storage and back-up capacity.
The role of bioenergy in DHC networks has been analyzed in many Nordic system level studies, but the focus is typically in large-scale systems and how existing large bio-CHP plants interact with the increasing amount of VRE and heat pumps. For example Levihn [34] showed how bio-CHP plants can provide flexibility in Stockholm's DHC network by shifting between back pressure and turbine bypass modes. Rinne & Syri [35] concluded that by increasing the heat storage capacity in Finnish DH networks to almost six fold from the current capacity the CHP plants can be efficiently used to balance wind power fluctuations, when the wind power share increases to 24%. However, the integration of variable heat production into the smaller municipal DH networks, which typically include HOB instead of CHP plant is a less studied topic.
The main challenge in wider solar heat utilization lies in misalignment between heat supply and demand. To balance this difference a solar thermal system is commonly coupled with a diurnal TES. Tian et al. [25] concluded that a diurnal TES enables 20–25% annual solar share, while large seasonal TES can increase the share up to 30–50% economically. A group of previous studies on hybrid heating networks has shown that solar thermal energy with significant annual contribution can be integrated to the DH network. Rosato et al. [36] studied different configurations of solar energy and seasonal borehole TES to supply heat to a small residential area and school buildings in Italy. Independently from the technology choice and control logic of the solar circuit, the system decreased the primary energy consumption, carbon dioxide (CO2) emissions and the operating costs compared to a conventional reference heating plant. Bauer et al. [37] analyzed existing as well as planned small-scale SDH systems (62–320 m2) with seasonal TES (183–800 m3) in Spain, Poland and Germany and concluded that small-scale systems with seasonal TES can be a well-functioning alternative to larger ones. The use of heat pumps is recommended to increase the system efficiency, which often is the challenge in small-scale systems. Buoro et al. [38] presented an optimization study for an industrial energy supply system in North Italy, coupling a SDH plant with TES and conventional power sources. The results showed that cost-optimal sized solar field produces 55–60% of the annual heat demand with weekly charging/discharging of the TES, though this leads to surplus heat during the summer.
There are both studies and existing examples of solar communities at high latitudes in heating-dominated climate. The Drake Landing Solar Community (DLSC) established in 2007 in Alberta, Canada is a well-known project in the field through achievement of 97% annual solar share in residential DH network in cold climate [39]. Flynn & Sirén [40] demonstrated the impacts of location changes and design modifications on a system similar to DLSC in five different locations. The findings indicate that solar share of over 95% is achievable in each location, while seasonal TES is more important in cold climates than in sunny and warm climates. The most challenging setting for the system is in Helsinki, Finland due to high heating demand and low level of irradiation in winter. Renaldi & Friedrich [41] expanded the previous study to UK and concluded that solar share of close to 80% is possible, but leads to LCoE of almost three times the state-of-the-art heating systems. Hirvonen et al. [42] performed techno-economic optimization for four different solar community sizes in Finland and concluded that 65–92% of the heat can be covered by the combination of solar heat and heat pump. Ur Rehman et al. [43] showed similar results by ending up to 53–81% renewable energy fraction in different solar community configurations in Finnish conditions. Common strategy for the above mentioned cases is to cover significant fraction of annual heat demand through seasonal TES.
Only a very limited number of studies are available concerning explicitly economics of integrating solar heat and biomass in a DH network. Lizana et al. [24] compared two DH systems in a rural municipality in South Spain, a biomass-only DH system and a SDH system with a seasonal TES and biomass support system, through a cost-benefit analysis. The study concluded that solar share of over 75% gives an optimal performance in Southern Europe in terms of generation costs. The biomass-only system outperforms the solar hybrid system, but the result is sensitive to biomass price variations e.g. due to competition of resource. Huang et al. [27] focused on the economic analyses of combinations of solar heat with air source heat pumps, ground-source heat pumps, gas boilers, and gas boilers and seasonal TES in China. It was concluded that in the case of biomass price of over 139 €/ton (CNY/Euro exchange rate of 7.8899) solar thermal integration lowers the levelized cost of heat (LCoH) of the system. According to Danish Energy Agency [44], the LCoH based on wood pellets, wood chips and straw is lower than 35 €/MWh, while the price of solar heat is stated to vary between 20 and 40 €/MWh [25].
The above mentioned literature reveals that there is a lack of studies about solar heat integration to municipal bioenergy dominated DH networks in cold climates, where heating demand concentrates on winter period, while solar irradiation is mostly available during the summer period. Most of the previous studies have focused on fossil fuel replacement in DH network by pursuing a high annual solar share through seasonal TES. While the studies have shown the viability of such solutions in many conditions, there is less knowledge about solar integration to existing RES-dominated systems without a seasonal TES. Especially in the Nordic countries, solid biomass is commonly used as a fuel for DH production. In Finland, 36% of DH is supplied from biomass [45], while in Sweden the number is 46% [33]. However, sustainable biomass is a limited resource, and increasing demand for bioenergy for the displacement of fossil energy sources in various applications and sectors increases pressure for biomass availability [14,23]. In bioenergy dominated networks, fuel oil and natural gas are typically used in peak boilers to supply demand peaks during the winter and to supply low heat demands during the summer. Solar heat could decrease both the remaining fossil fuel oil and biomass consumption, yet, the integration taking into account possible inflexibilities of solid biomass boiler has not been properly studied.
In order to advance quantitative knowledge of implications of adding solar heat to an existing DH network and to find pathways for cutting remaining carbon emissions this paper presents a case study based on dynamic simulation and cost and emission analysis methods. The case study is based on existing DH network of Suonenjoki municipality, Finland and actual data, but the developed model is replicable to other locations as well. The model is used to examine a novel case of how to use solar thermal energy in the decarbonization of a heating system that already includes a large share (over 90%) of biomass. The integration strategy aims at cost-efficiency by taking use of existing biomass infrastructure instead of installing a new TES system. In particular, the objective is to find solutions that minimize the support fuel oil consumption and emissions during the summer period caused by the low load of the system.
Section snippets
Case study definition
The district heating network of the central area of Suonenjoki municipality, located in the Central Finland, is used as a case study in this work (see Appendix A). The heating degree days in the location are 4436 Kday (2016) with indoor temperature of 17 °C [46]. The case study covers 66 district heated buildings, and the area is part of the wider DH network of the municipality. The main heat source of the network is a 10 MW forest wood chip HOB (including flue gas condenser), which can also
Introducing solar heat without limitations to the DH system
Solar heat can be used to displace fuel oil consumption from the DH system during the summertime. An intuitive approach is to dimension the solar field capacity to the corresponding summertime heat load. Simulation case SH-600 represents this approach. Annual simulation results regarding the heat source mix used in the DH system are given in Fig. 4a and the heat source utilization during two example days is given in Fig. 4b.
In the SH-600 case, solar heat production never exceeds heat
Conclusions
Different solar heat integration strategies to phase out fuel oil use and minimize greenhouse gas emissions in the existing municipal DH network in central Finland were studied. While most of the existing studies aim towards high annual solar shares and extensive replacement of fossil fuels through seasonal TES integration, the primary objective of this study was to replace the remaining fossil fuel use during the summer period in a bioenergy dominated (over 90%) system in cold climate. As an
CRediT authorship contribution statement
Elina Mäki: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Writing – original draft, Writing – review & editing. Lotta Kannari: Data curation, Formal analysis, Investigation, Methodology, Software, Validation. Ilkka Hannula: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Visualization, Writing – review & editing. Jari Shemeikka: Funding acquisition, Project administration, Resources.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by research project “VaBiSys − Value-optimised use of biomass in a flexible energy infrastructure” funded under the ERA-Net Bioenergy network and BESTF3 ERA-Net co-fund mechanism (reference BEN11-17-28); research project “MODER − Mobilization of innovative design tools for refurbishing of buildings at district level” (grant number 680447) funded by European Commission under Horizon 2020 research and innovation programme. The authors gratefully thank Savon Voima Oyj for
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Present address: International Energy Agency, 9 rue de la Fédération, 75739 Paris, France, [email protected] Contributions to this paper were completed before joining the IEA, and therefore this paper does not necessarily reflect the views of the IEA Secretariat nor of its individual Member countries.