Solution for the future smart energy system: A polygeneration plant based on reversible solid oxide cells and biomass gasification producing either electrofuel or power
Graphical abstract
Introduction
The Danish energy system is continuing to develop to be independent of fossil fuels by 2050 [1]. Moreover, the goal is to decrease greenhouse gas emissions significantly in future decades [2]. This is a challenge for the energy system, as transformation from fossil to a more sustainable energy system is likely to increase the utilization of intermittent energy sources such as wind. The energy system needs to be flexible and adaptive for the effective use of intermittent energy sources [3], [4]. The increased penetration of wind has and will continue to change the operation time and hourly profile loads of thermal power plants significantly, which will severely impact their economic feasibility [4]. However, the role of thermal power plants can still be important in balancing demand and supply; to phase out fossil fuels cost effectively using intermittent resources, a smart system must be coupled with a flexible thermal plant that can produce additional electricity when required or store electricity when production is high but demand is low. Mathiesen et al. [5] stated that electricity storage technologies such as batteries serve a function in the future energy system for managing short-term fluctuations, but do not play a large-scale role in handling annual fluctuations in a system with electricity produced by intermittent resources. In a review article on energy storage technologies by Luo et al. [6], hydrogen production and fuel cells are regarded as an option for dealing with fluctuating production from renewables, as electricity can be used in a water electrolyser to generate hydrogen when electricity demand is low (low market price), and hydrogen can be used in a fuel cell to produce electricity when demand is high (high market price). Another method could be the use of a reversible operation of solid oxide cells (SOCs) under electrolysis and fuel cell modes [7], which will decrease the number of components required, thereby increasing economic feasibility. Another means could be the use of hydrogen to create synthetic natural gas (SNG), which can be stored in existing natural gas grids, where the carbon source for SNG could be syngas from gasified biomass. An important advantage of generating SNG compared to hydrogen is that SNG has a higher energy density per volume, which makes it easier to store and use in the transportation sector.
Lund and Mathiesen [8] provided a 100% renewable energy system case analysis for Denmark in the years 2030 and 2050, which demonstrated its feasibility. However, the biomass demand is potentially high for its sustainable utilization. Mathiesen et al. [5] further illustrated that a 100% renewable energy system can be achieved with sustainable biomass consumption by creating a smart energy system. In this model, the electricity, heating, and transportation sectors are merged by producing electrofuels using electrolysis based on electricity from intermittent resources and biomass gasification, supplying electricity and additional heat through combined heat and power plants. Connolly et al. [9] introduced transition steps toward a 2050 smart energy system for Europe, where the final steps involve producing renewable electrofuels to provide new transport fuels, replace coal and oil, and finally, replace natural gas. Thermochemical and biochemical conversions are the two important bioenergy technologies for converting biomass, whereas combustion, pyrolysis, and gasification are the main options for thermochemical conversion [10]. The integration of gasification and electrolysis for fuel synthesis is not a novel approach, and has recently been researched in detail [11], [12], [13], [14], [15]. However, this research did not include a reversible system that can choose between electricity production and consumption for bio-SNG production. The purpose of this study is to introduce a system that can provide a solution for electricity markets with intermittent production in a future energy system by demonstrating the relevance of the ability to change operation between producing and using electricity based on market price. The proposed system is analyzed using the predicted hourly price duration curve in the Danish electricity market for the years 2025 and 2035. The hypothesis is that such a system can operate with a high capacity factor by enabling full operation all year round. This will increase the economic feasibility of future energy systems compared to using stand-alone gasifier and electrolyser plants.
A biomass-based polygeneration system concept is proposed, which includes storage of electricity from fluctuating sources. The system produces heat, electricity, and SNG (bio-SNG), and stores electricity by producing bio-SNG when electricity market prices are low and producing electricity when market prices are high. Fig. 1 illustrates the main inputs and outputs of the proposed system.
This system is divided into two operation modes, namely the electricity production and bio-SNG production (electricity storage) modes. The main components of the system are depicted in Fig. 2, Fig. 3 for the electricity and bio-SNG production modes, respectively. This model is designed to operate in an energy system with highly fluctuating renewable energy input (for example, wind) by taking advantage of the varying electricity prices, providing electricity and heat when required, and producing renewable electrofuel (bio-SNG) when electricity prices are low. This system can serve as a crucial component of a future smart energy system, as suggested by Mathiesen et al. [5] and Connolly et al. [9].
The analytical framework is constructed based on thermodynamic modeling using DNA software, which is a component-based thermodynamic modeling and simulation tool [16]. DNA is open-source software developed in the Thermal Energy Section of the Mechanical Engineering Department at DTU (an in-depth description is provided by Brian Elmegaard [17]). Mass and energy conservation is automatically included in DNA, providing the foundation for electricity and bio-SNG production energy analysis by the system. The techno-economic analysis was modeled in Python using process data from DNA, and the analysis was used to determine the total revenues required and net present value, given a range of bio-SNG and electricity prices. The marginal cost of operation for both production modes is calculated by fuel and other running costs, along with the electricity market spot price, which determines the yearly running time of the system (capacity factor). To determine the yearly running time, the current (2016) and predicted power price cumulative curves are used, along with district heat price scenarios over a range of possible bio-SNG prices.
Section snippets
Design of energy system
As shown in Fig. 2, the system uses wood chips in an air-blown gasifier, and the produced syngas is supplied to a reversible solid oxide cell operating as a fuel cell (SOFC) to produce electricity. The unconverted fuel from the SOFC is subsequently used in a gas engine to increase electricity production further. Moreover, a significant amount of heat is generated by the system, which is used to produce district heating. The system is thus fueled by wood chips, producing electricity as its
Results
The results of the thermodynamic modeling of the two operation modes are presented below. First, the energy system simulation results are presented, followed by the techno-economic analysis. Then, the marginal cost and mode of operation are presented for the given electricity and bio-SNG prices, along with the operation time in each mode. Finally, the capacity factor and NPV of the polygeneration system are provided, based on scenario analysis and in reference to a single operation mode.
Conclusion
This article presented a study on the thermodynamic modeling and simulation of a novel polygeneration plant, along with a techno-economic analysis. The results demonstrated that the hypothesis stands as this system can operate with a high capacity factor in the future Danish electricity market. Furthermore, the results indicated that economic feasibility is greater compared to using stand-alone gasifier and electrolyser plants for electrofuel production.
Based on the results of the study,
Acknowledgements
The authors would like to thank the ForskEL programme for funding this research.
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