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From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage
Anton A. Kiss, Edwin Zondervan, Richard Lakerveld, Leyla Özkan (Eds.) Proceedings of the 29th European Symposium on Computer Aided Process Engineering June 16th to 19th , 2019, Eindhoven, The Netherlands. © 2019 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/B978-0-12-818634-3.50292-7
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage Francesco Baldia* , Alain Azzia and François Maréchala a Laboratory of Industrial processes and energy systems engineering, École Polytechnique
Fédérale de Lausanne, EPFL Valais-Wallis, Rue de l’Industrie 17, 1950 Sion, Switzerland francesco.baldi@epfl.ch
Abstract The stochastic and non-controllable nature of most of renewable energy sources makes it necessary to include extensive use of energy storage in national grids to overcome periods with low availability. Hence, effective energy storage technologies will be requiredfor achieving a 100% renewable energy system. Currently used technologies are only partly suitable for this task. Batteries are efficient but expensive, and mostly suitable for daily storage. Hydrogen has a significant potential, but suffers from a low energy density and difficulties in handling and transportation. Among different potential solutions, ammonia was often pointed out as a high-density and lowcost hydrogen carrier. In this paper, we analyze the efficiency of an ammonia-based pathway for the storage of excess energy from renewable energy sources, its transportation, and its final use. As a case study, we consider the use on board of a urban car transport vessel as the user of the stored energy. The energy efficiency and cost of the whole chain, from the production at the wind farm until its use on board of the ferry, is evaluated and compared with competitive alternatives, namely batteries and hydrogen storage. The results show that, while not being a game-changer, the use of ammonia as mean for storing and transporting excess energy from renewable power plants is viable and, in combination with other storage systems, contributes to a relevant part of the share of total installed storage capacity in the most cost-effective solution. Keywords: energy storage, renewable energy, ammonia, fuel cells
1. Introduction The challenge of making human activities environmentally sustainable is one of the most important that humanity will have to face in the coming years. While the solution will be provided by a combination of different technological developments, it is widely recognized that renewable energy sources will take a major share of the task. Given the stochastic and non-controllable nature of most of renewable energy sources, improving the performance of energy storage represents one of the most important challenges to face for achieving a 100% renewable energy system. Batteries, while being the most efficient storage technology, suffer from limitations in energy density and high cost. Hydrogen generated by electrolysis, while still providing a reasonably high round-trip
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efficiency, has limitations in energy density and in handling. As a solution for long-term storage and for transportation, various authors have proposed the conversion of hydrogen to conventional fuels, such as syngas, methane, methanol, and Diesel, but none of these solutions would prove to be carbon neutral, unless the carbon is originated by biomass, or by carbon capture and storage. The use of ammonia as fuel can provide a valid alternative to carbon-based fuels. Ammonia has a relatively energy density in its liquid phase, that can be achieved at conditions relatively close to ambient conditions. For this reason, we believe that there is an interest in proposing ammonia as an energy vector, and as a mean for energy storage. In this paper, we aim at investigating the potential role of ammonia as a way to store renewable energy that is produced in excess during low-demand periods. We address this by looking at a specific case study, based on the hypothesis to use the extra energy to propel ferries. The study aims at comparing existing storage technologies (batteries and hydrogen) with the potential case of ammonia.
2. Method In order to investigate the potential role of ammonia as means for storing renewable energy during periods of excessive availability, in this paper we refer to the case of an offshore wind farm of a rated power of 200 MW. The excess energy can either be curtailed, and hence lost, or stored and then transferred to a harbor, where it is assumed that a car ferry will be the final user. The process of storage and transportation of the excess energy is allowed to take different ways, as shown in Figure 1: Electric way : Excess energy is stored in batteries, and then sent to shore via existing power lines. The power transmission is allowed whenever there is no excess energy generation. Hydrogen way : Excess energy is used in an electrolyser to generate hydrogen, that is stored in either compressed or liquid form. It is here assumed that the hydrogen is stored in container-sized tanks, that can then be loaded on supply vessels that regularly serve the wind farm. A bi-weekly frequency is assumed for the purpose of this study, as reported by CIT. Ammonia way : Excess energy is used to generate hydrogen from water in an electrolyser, and nitrogen from air in a cryogenic air separation unit. Hydrogen and nitrogen are then used in an ammonia synthesis plant (here assumed based on the Haber-Bosch technology) that converts them to ammonia, that is then stored in liquid form at low temperature. The transportation is assumed to happen in the same way as assumed for the hydrogen way. Once delivered to the port, the different fuels can be loaded on board of the ferry that operates on daily round trips. Electrical energy can be stored onboard in batteries, and directly used to power electric motor. Hydrogen is assumed to be used in a proton exchange membrane fuel cell (PEMFC), a well-developed technology extensively tested for
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage
1749
Figure 1: Representation of the superstructure for the optimization model
Name
Location
Cinv
Unit
η
Unit
EUR kWH2 EUR kWH2 EUR kWH2 EUR
0.731
kWel kWH2 kWel kWH2 kWel kWH2 kW hel kgN2
Electrolyser
WF
4001
H2 Compression plant
WF
1802
H2 Liquefaction plant
WF
6504
N2 Separation plant
WF
14501
NH3 Synthesis plant
WF
30001
1 Ikäheimo
kgH 2 h
EUR kgNH3 /h
0.913 0.645 0.111 0.641
kW hel kgNH3
et al. (2018), 2 Law et al. (2013), 3 Tzimas et al. (2003), 4 Kelly (2007), 5 Gardiner (2009)
Table 1: Numerical assumptions employed in the study
use with hydrogen as fuel, also in maritime applications CIT. Ammonia is assumed to be used in a solid oxide fuel cell (SOFC); while the technology of SOFCs is less mature when compared to PEMFCs, particularly when powered by ammonia, recent tests have shown that there is no conceptual, or practical obstacle to a wider adoption of this technology CIT. The assumptions related to unit cost and efficiency are summarized in Table XXX. We assume that the maintenance cost does not significantly vary between different options and technologies and that, hence, can be excluded from the analysis. The evolution of the excessive power generation over the year is represented using to the processed data provided by Zerrahn et al. (2018), originally retrieved in raw format from ?, and refer to the German electrical grid for the year 2014. This dataset provides hourly values for the curtailed energy from the combination of all renewable power plants. The implicit assumption of using this data in this paper is that the excess power is redistributed among different plants based on the installed power capacity. While this can be considered as a strong assumption, this approach provides a relatively accurate representation of the size and frequency of power curtailment in a European electrical grid. A representation
F. Baldi et al.
1750
Figure 2: Wind curtailment power, probability of occurrence of the resulting frequency of different curtailment powers is provided in Figure XXX. Both the excess power and the power demand of the ferry are represented with a two-hour definition. As this would require a total of 4320 time steps for one year of operation, we assume that yearly operations can be summarized with a total number of four "reference periods" of the duration of two weeks, where the choice of the period length is based on the frequency available for transporting fuel from the wind farm to shore. The four reference periods where defined as follows: • Maximum cumulated excess energy (for dimensioning the storage), occurring once per year • Maximum peak excess power (for dimensioning the conversion units), occurring once per year • Reference period at low cumulated excess energy, occurring 15 times per year • Reference period at high cumulated excess energy, occurring 9 times per year
3. Results and discussion The results for the optimal annualized costs in the different cases are provided in Figure 3. The option of using only electric energy storage in the form of batteries is clearly the least convenient case: as a consequence of the high specific investment cost, the optimizer chooses to reduce the total storage installed capacity, hence increasing the operational costs related to energy curtailment. The results also show that, for the selected case study, liquid hydrogen storage is more competitive in comparison to compressed hydrogen storage. Both the increased energy cost for liquefaction and the relatively high investment
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage
1751
Figure 3: Comparison of operational, investment, and total costs for the different investigated cases cost of the hydrogen liquefaction plant are offset by the lower investment cost for hydrogen storage. When ammonia comes into play, a combination of effects can be observed. The use of ammonia alone as storage option is less profitable compared to the case of liquid hydrogen storage. This is mostly due to the high specific investment cost of the ammonia synthesis plant, which in this case is the largest contributor to the investment cost (32%), while the cost of the storage is only marginal (less than 1%). On the other hand, when other storage possibilities are included, the ammonia production plant is downsized (the installed capacity is reduced by 38%), and liquid hydrogen is used for intermediate storage. The detailed cost share in this case is shown in Figure 4: the electrolyser, the SOFC and the ammonia synthesis plant all contribute with 20-25% of the investment costs, while the use of a battery for part of the onboard power generation is favored because of the lower operational costs, particularly when energy is bought from the energy market.
4. Conclusion
References M. Gardiner, 2009. Energy requirements for hydrogen gas compression and liquefaction as related to storage needs. Technical Report 9013, US Department of Energy, United States. J. Ikäheimo, J. Kiviluoma, R. Weiss, H. Holttinen, 2018. Power-to-ammonia in future North European 100 % renewable power and heat system. International Journal of Hydrogen Energy 43 (36), 17295–17308. B. Kelly, 2007. Liquefaction and pipeline costs. K. Law, J. Rosenfeld, V. Han, M. Chan, H. Chiang, J. Leonard, 3 2013. U.s. department of energy hydrogen storage cost analysis. Tech. rep. E. Tzimas, C. Filiou, S. Peteves, J.-B. Veyret, 2003. Hydrogen storage: state-of-the-art and future perspective. Tech. Rep. 20995, European Commission, Joint Research Centre (JRC), Directorate General (DG), Institute for Energy, Petten, The Netherlands.
F. Baldi et al.
1752
Figure 4: Investment cost breakdown, full case A. Zerrahn, W.-P. Schill, C. Kemfert, Sep. 2018. On the economics of electrical storage for variable renewable energy sources. European Economic Review 108, 259–279.
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage Francesco Baldia* , Alain Azzia and François Maréchala a Laboratory of Industrial processes and energy systems engineering, École Polytechnique
Fédérale de Lausanne, EPFL Valais-Wallis, Rue de l’Industrie 17, 1950 Sion, Switzerland francesco.baldi@epfl.ch
Abstract The stochastic and non-controllable nature of most of renewable energy sources makes it necessary to include extensive use of energy storage in national grids to overcome periods with low availability. Hence, effective energy storage technologies will be requiredfor achieving a 100% renewable energy system. Currently used technologies are only partly suitable for this task. Batteries are efficient but expensive, and mostly suitable for daily storage. Hydrogen has a significant potential, but suffers from a low energy density and difficulties in handling and transportation. Among different potential solutions, ammonia was often pointed out as a high-density and lowcost hydrogen carrier. In this paper, we analyze the efficiency of an ammonia-based pathway for the storage of excess energy from renewable energy sources, its transportation, and its final use. As a case study, we consider the use on board of a urban car transport vessel as the user of the stored energy. The energy efficiency and cost of the whole chain, from the production at the wind farm until its use on board of the ferry, is evaluated and compared with competitive alternatives, namely batteries and hydrogen storage. The results show that, while not being a game-changer, the use of ammonia as mean for storing and transporting excess energy from renewable power plants is viable and, in combination with other storage systems, contributes to a relevant part of the share of total installed storage capacity in the most cost-effective solution. Keywords: energy storage, renewable energy, ammonia, fuel cells
1. Introduction The challenge of making human activities environmentally sustainable is one of the most important that humanity will have to face in the coming years. While the solution will be provided by a combination of different technological developments, it is widely recognized that renewable energy sources will take a major share of the task. Given the stochastic and non-controllable nature of most of renewable energy sources, improving the performance of energy storage represents one of the most important challenges to face for achieving a 100% renewable energy system. Batteries, while being the most efficient storage technology, suffer from limitations in energy density and high cost. Hydrogen generated by electrolysis, while still providing a reasonably high round-trip
1748
F. Baldi et al.
efficiency, has limitations in energy density and in handling. As a solution for long-term storage and for transportation, various authors have proposed the conversion of hydrogen to conventional fuels, such as syngas, methane, methanol, and Diesel, but none of these solutions would prove to be carbon neutral, unless the carbon is originated by biomass, or by carbon capture and storage. The use of ammonia as fuel can provide a valid alternative to carbon-based fuels. Ammonia has a relatively energy density in its liquid phase, that can be achieved at conditions relatively close to ambient conditions. For this reason, we believe that there is an interest in proposing ammonia as an energy vector, and as a mean for energy storage. In this paper, we aim at investigating the potential role of ammonia as a way to store renewable energy that is produced in excess during low-demand periods. We address this by looking at a specific case study, based on the hypothesis to use the extra energy to propel ferries. The study aims at comparing existing storage technologies (batteries and hydrogen) with the potential case of ammonia.
2. Method In order to investigate the potential role of ammonia as means for storing renewable energy during periods of excessive availability, in this paper we refer to the case of an offshore wind farm of a rated power of 200 MW. The excess energy can either be curtailed, and hence lost, or stored and then transferred to a harbor, where it is assumed that a car ferry will be the final user. The process of storage and transportation of the excess energy is allowed to take different ways, as shown in Figure 1: Electric way : Excess energy is stored in batteries, and then sent to shore via existing power lines. The power transmission is allowed whenever there is no excess energy generation. Hydrogen way : Excess energy is used in an electrolyser to generate hydrogen, that is stored in either compressed or liquid form. It is here assumed that the hydrogen is stored in container-sized tanks, that can then be loaded on supply vessels that regularly serve the wind farm. A bi-weekly frequency is assumed for the purpose of this study, as reported by CIT. Ammonia way : Excess energy is used to generate hydrogen from water in an electrolyser, and nitrogen from air in a cryogenic air separation unit. Hydrogen and nitrogen are then used in an ammonia synthesis plant (here assumed based on the Haber-Bosch technology) that converts them to ammonia, that is then stored in liquid form at low temperature. The transportation is assumed to happen in the same way as assumed for the hydrogen way. Once delivered to the port, the different fuels can be loaded on board of the ferry that operates on daily round trips. Electrical energy can be stored onboard in batteries, and directly used to power electric motor. Hydrogen is assumed to be used in a proton exchange membrane fuel cell (PEMFC), a well-developed technology extensively tested for
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage
1749
Figure 1: Representation of the superstructure for the optimization model
Name
Location
Cinv
Unit
η
Unit
EUR kWH2 EUR kWH2 EUR kWH2 EUR
0.731
kWel kWH2 kWel kWH2 kWel kWH2 kW hel kgN2
Electrolyser
WF
4001
H2 Compression plant
WF
1802
H2 Liquefaction plant
WF
6504
N2 Separation plant
WF
14501
NH3 Synthesis plant
WF
30001
1 Ikäheimo
kgH 2 h
EUR kgNH3 /h
0.913 0.645 0.111 0.641
kW hel kgNH3
et al. (2018), 2 Law et al. (2013), 3 Tzimas et al. (2003), 4 Kelly (2007), 5 Gardiner (2009)
Table 1: Numerical assumptions employed in the study
use with hydrogen as fuel, also in maritime applications CIT. Ammonia is assumed to be used in a solid oxide fuel cell (SOFC); while the technology of SOFCs is less mature when compared to PEMFCs, particularly when powered by ammonia, recent tests have shown that there is no conceptual, or practical obstacle to a wider adoption of this technology CIT. The assumptions related to unit cost and efficiency are summarized in Table XXX. We assume that the maintenance cost does not significantly vary between different options and technologies and that, hence, can be excluded from the analysis. The evolution of the excessive power generation over the year is represented using to the processed data provided by Zerrahn et al. (2018), originally retrieved in raw format from ?, and refer to the German electrical grid for the year 2014. This dataset provides hourly values for the curtailed energy from the combination of all renewable power plants. The implicit assumption of using this data in this paper is that the excess power is redistributed among different plants based on the installed power capacity. While this can be considered as a strong assumption, this approach provides a relatively accurate representation of the size and frequency of power curtailment in a European electrical grid. A representation
F. Baldi et al.
1750
Figure 2: Wind curtailment power, probability of occurrence of the resulting frequency of different curtailment powers is provided in Figure XXX. Both the excess power and the power demand of the ferry are represented with a two-hour definition. As this would require a total of 4320 time steps for one year of operation, we assume that yearly operations can be summarized with a total number of four "reference periods" of the duration of two weeks, where the choice of the period length is based on the frequency available for transporting fuel from the wind farm to shore. The four reference periods where defined as follows: • Maximum cumulated excess energy (for dimensioning the storage), occurring once per year • Maximum peak excess power (for dimensioning the conversion units), occurring once per year • Reference period at low cumulated excess energy, occurring 15 times per year • Reference period at high cumulated excess energy, occurring 9 times per year
3. Results and discussion The results for the optimal annualized costs in the different cases are provided in Figure 3. The option of using only electric energy storage in the form of batteries is clearly the least convenient case: as a consequence of the high specific investment cost, the optimizer chooses to reduce the total storage installed capacity, hence increasing the operational costs related to energy curtailment. The results also show that, for the selected case study, liquid hydrogen storage is more competitive in comparison to compressed hydrogen storage. Both the increased energy cost for liquefaction and the relatively high investment
From renewable energy to ship fuel: ammonia as an energy vector and mean for energy storage
1751
Figure 3: Comparison of operational, investment, and total costs for the different investigated cases cost of the hydrogen liquefaction plant are offset by the lower investment cost for hydrogen storage. When ammonia comes into play, a combination of effects can be observed. The use of ammonia alone as storage option is less profitable compared to the case of liquid hydrogen storage. This is mostly due to the high specific investment cost of the ammonia synthesis plant, which in this case is the largest contributor to the investment cost (32%), while the cost of the storage is only marginal (less than 1%). On the other hand, when other storage possibilities are included, the ammonia production plant is downsized (the installed capacity is reduced by 38%), and liquid hydrogen is used for intermediate storage. The detailed cost share in this case is shown in Figure 4: the electrolyser, the SOFC and the ammonia synthesis plant all contribute with 20-25% of the investment costs, while the use of a battery for part of the onboard power generation is favored because of the lower operational costs, particularly when energy is bought from the energy market.
4. Conclusion
References M. Gardiner, 2009. Energy requirements for hydrogen gas compression and liquefaction as related to storage needs. Technical Report 9013, US Department of Energy, United States. J. Ikäheimo, J. Kiviluoma, R. Weiss, H. Holttinen, 2018. Power-to-ammonia in future North European 100 % renewable power and heat system. International Journal of Hydrogen Energy 43 (36), 17295–17308. B. Kelly, 2007. Liquefaction and pipeline costs. K. Law, J. Rosenfeld, V. Han, M. Chan, H. Chiang, J. Leonard, 3 2013. U.s. department of energy hydrogen storage cost analysis. Tech. rep. E. Tzimas, C. Filiou, S. Peteves, J.-B. Veyret, 2003. Hydrogen storage: state-of-the-art and future perspective. Tech. Rep. 20995, European Commission, Joint Research Centre (JRC), Directorate General (DG), Institute for Energy, Petten, The Netherlands.
F. Baldi et al.
1752
Figure 4: Investment cost breakdown, full case A. Zerrahn, W.-P. Schill, C. Kemfert, Sep. 2018. On the economics of electrical storage for variable renewable energy sources. European Economic Review 108, 259–279.