Microsoft Word - PRES22_0064.docx DOI: 10.3303/CET2294078 Paper Received: 19 April 2022; Revised: 05 May 2022; Accepted: 11 May 2022 Please cite this article as: Šulc R., Ditl P., 2022, The Potential of Oxygen -Pressure Swing Adsorption Unit Connected with Electricity Storage System, Chemical Engineering Transactions, 94, 469-474 DOI:10.3303/CET2294078 A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 94, 2022 The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš, Sandro Nižetić Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-93-8; ISSN 2283-9216 The Potential of Oxygen-Pressure Swing Adsorption Unit Connected with Electricity Storage System Radek Šulc*, Pavel Ditl Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Process Engineering, Technická 4, Prague, Czech Republic Radek.Sulc@fs.cvut.cz The pressure swing adsorption (PSA) units are widely used as an oxygen source. The storage of pressurized gaseous oxygen is limited by the capacity of a pressure vessel. The increasing share of renewable electricity sources (RES) causes intraday electricity price fluctuations. These fluctuations can be an opportunity to improve the economy of a plant. This paper aims to analyze the potential of a PSA unit connected to the battery energy storage system (BESS) for more effective on-site oxygen production. The analysis was carried out for the Czech Republic, Germany, and Denmark. These countries differ significantly in the energy mix. The theoretical potential of BESS installation and use in electricity price peak was found to be around 9 - 16 % of cost-saving on average compared with the daily operation of PSA unit when the off-peak average electricity price was from 95 to 91 % of the daily average electricity price respectively. Widening the price gap due to increasing RES share, the potential is growing. 1. Introduction The Pressure Swing Adsorption (PSA) units are widely used as oxygen sources where oxygen is produced in gaseous form. Start-up time taking minutes is an undeniable advantage of PSA technology compared to cryogenic air separation start-up time taking hours or days. The capacity of a pressure vessel limits the storage of pressurized gaseous oxygen. The increasing share of renewable electricity causes intraday electricity price fluctuations. These fluctuations can be an opportunity to improve the economy of a plant (Miller et al., 2008) depending on market conditions (Cao et al., 2017), and to accumulate electricity in the form of liquefied products (Casperi et al., 2019ab). Casperi et al. (2019b) reported an improvement of 14 % in comparison with quasi- stationary scheduling for the designed flexible air separation unit (ASU) with an integrated liquefaction cycle and liquid assist operation Casperi et al. (2019a). Šulc and Ditl (2021a) analyzed two options for cost-saving: i) liquefied oxygen (LOX) supply at electricity price peak, and ii) liquid oxygen energy storage (LOES). The cold energy needed for oxygen liquefaction was obtained utilizing liquefied nitrogen (LIN) delivered from a large air separation unit. The on-site oxygen liquefaction and storage were found to be effective only for PSA units with double compression when the electricity price in the storage period was approximately three-four times lower than the daily average electricity price. This paper aims to analyze the potential of a PSA unit connected to the battery energy storage system (BESS) for more effective on-site oxygen production. The analysis was carried out for the Czech Republic, the Federal Republic of Germany, and the Kingdom of Denmark. These countries differ significantly in the energy mix. The effect of intra-day electricity price fluctuations and energy storage system costs will be taken into account during the economic analysis. For comparison, the data used by Šulc and Ditl (2021a) were applied. 1.1 Battery Energy Storage System The electric energy time-shift is one of the grid applications of battery energy storage systems. Electric energy time-shift involves purchasing inexpensive electric energy, available during periods when prices or system marginal costs are low, to charge the storage system so that the stored energy can be used or sold at a later time when the price or costs are high. This application has also a potential for CO2 emission reduction (Kim et al., 2018). The BESS consists of: i) the battery pack, which connects multiple cells with an appropriate voltage 469 and capacity, ii) the battery management system (BMS) which protects the cells from harmful operation to achieve reliable and safe operation, and iii) the battery thermal management system (B-TMS) which controls the temperature of the cells according to their specifications (Kim et al., 2018). The following main parameters affect the BESS sizing:  Nominal capacity represents the amount of energy (as A h or W h) that the battery can nominally deliver from a fully charged state at a nominal discharge current. Capacity depends on C-rate and temperature.  State of Charge (SOC, %) represents the actual battery capacity as a percentage of nominal capacity.  Depth of discharge (DOD, %) represents the percentage of battery capacity that has been discharged in a given cycle, i.e., defines the usable capacity. The DOD is expressed as a percentage of nominal capacity.  Discharge/charge rate (C-rate,1) is the measure of the current in which a battery is charged or discharged.  Cycle Life represents the number of discharge-charge cycles of the battery before it loses the required performance. Cycle life is affected by the rate and depth of cycles. The higher the DOD, the lower the cycle life.  Total round-trip efficiency represents the ratio of energy delivered from BESS and the energy supplied to BESS. It takes into account the energy losses from power conversions and auxiliary loads associated with BESS operating. 1.2. Intra-day electricity prices The electricity cost is a major cost item of oxygen production by pressure swing adsorption. For the analysis, the day-ahead prices reported by ENTSO-E Transparency Platform were used. No taxes (VAT and recoverable taxes) and levies were not taken into account. The prices vary during the year, months, and days. Therefore, the data for the randomly selected 2nd Wednesday in January, April, July, and October of the year 2020 respecting the winter, spring, summer, and autumn seasons, respectively, were overtaken. The analysis was carried out for the Czech Republic, the Federal Republic of Germany, and the Kingdom of Denmark. a) January 8, 2020 b) April 8, 2020 c) July 8, 2020 d) October 7, 2020 Figure 1: Day-ahead prices for the Czech Republic, Germany, and Denmark: a) January 8, 2020; b) April 8, 2020; c) July 8, 2020; d) October 7, 2020 470 These countries differ significantly in the energy mix. The Czech Republic generated an average approximately 36 % in nuclear power plants, 49 % in thermal power plants, and 14 % from renewable energy sources on selected days. Unlike this, Denmark generated an average of approximately 25 % of energy in thermal power plants, 75 % from renewable energy sources, and no energy was produced in nuclear power plants. The energy mix of Germany was between both countries. Germany generated an average approximately 12 % of energy in nuclear power plants, 40 % in thermal power plants, and 47 % from renewable energy sources. The day-ahead prices for January 8, April 8, July 8, and October 7 of the year 2020 and their comparison for the Czech Republic, Germany, and Denmark are presented in Figure 1. The significant morning and evening peaks are visible from 7 to 12 a.m. and from 7 to 9 p.m. In some cases, a significant price falls due to an excess of renewable energy sources. 1.3 Oxygen production by Pressure Swing Adsorption The analysis was carried out for on-site oxygen production using two-bed Pressure Swing Adsorption for 95 % purity and oxygen recovery characterized by the air ratio of 10 Nm3 Nm-3 producing 101 Nm3 h-1 of gaseous oxygen (GOX). For these conditions, Šulc and Ditl (2021b) reported the specific energy consumption of 0.805 kWh kgO2-1 and 0.728 kWh kgO2-1 for a single and dual compression, respectively. The data mentioned above were obtained for ambient air and compressed air at an outlet temperature of 30 °C and outlet pressure of 750 kPa (a) at the PSA unit inlet. The pressure losses of inter-and after-coolers were taken into account. 2. Methodology The electricity needed for the PSA unit is supplied by the grid during an off-peak period in which the electricity price is low. When the electricity price is lowest, the electricity from the grid is stored in a battery energy storage system during the charging period. During the electricity price peak, the electricity needed for the PSA unit is supplied by the charged BESS. The following assumptions were adopted for the following model for calculating the cost-saving: 1) the electricity cost is taken only into account; the investment cost and other operational costs such as depreciation, maintenance, etc. are not included, 2) the BESS efficiency is taken into account by the longer charging period compared to the discharging period. The daily average electricity price cel-daily (EUR MWh-1) was obtained numerically by the trapezoidal integration method. The off-peak average electricity price cel-offpeak (EUR MWh-1) and the charging average electricity price cel-charging (EUR MWh-1) were calculated analogically by the same procedure but for the off-peak period and the charging period, respectively. The 24 h operation, peak periods from 8 a.m. to 10 a.m. and from 7 p.m. to 10 p.m., and charging period from 0 a.m. to 6 a.m. were assumed. The electricity cost Cel-PSA-daily (EUR d-1) for the PSA unit operated for an operation time toper (h) was calculated: Cel-PSA-daily = cel-daily  ePSA toper , (1) where ePSA (MWh tO2-1) is the specific electricity consumption of the PSA unit. The electricity cost Cel-PSA-off-peak (EUR d-1) for PSA unit operated during off-peak period toff-peak (h) was calculated: Cel-PSA-off-peak = cel-off-peak ePSA toff-peak. (2) The electricity cost Cel-PSA-peak (EUR d-1) for PSA unit operated during the peak period tpeak (h) was calculated: Cel-PSA-peak = cel-charging ePSA tpeak. (3) Then, the gain(+)/loss(-) rate is estimated as it follows: gain/loss (%) = 100( Cel-PSA-off-peak + Cel-PSA-peak )/ Cel-PSA-daily . (4) The second option, the liquefied oxygen (LOX) supply in the electricity peak proposed by Šulc and Ditl (2021a) was also analyzed for comparison. In this case, the difference between the specific electricity consumption of LOX and GOX produced by the PSA unit is utilized for cost-saving. The oxygen needed for the process is supplied by the PSA unit during a period in which the electricity price is low. During the electricity price peak, the oxygen needed for the process is produced from LOX supplied from large ASU facilities continuously operated throughout the day. LOX is evaporated by ambient air. The LOX price is estimated based on the benchmark specific electricity consumption of LOX production and the daily averaged day-ahead electricity price. The LOX transportation cost is not included. The LOX cost CLOX-peak (EUR d-1) for LOX consumed during the peak period tpeak (h) was calculated: 471 Cel-LOX-peak = cel-daily  eLOX tpeak = cel-daily eLOX (toper - toff-peak ), (5) where eLOX (MWh tLOX-1) is the specific electricity consumption of LOX production. The benchmark specific electricity consumption of liquefied oxygen (LOX) is 638 kWh tLOX-1 (EIGA, 2019). Then, the gain(+)/loss(-) rate is estimated as it follows: gain/loss (%) = 100( Cel-PSA-off-peak + Cel-LOX-peak )/ Cel-PSA-daily . (6) 3. Results and discussion The cost-saving calculated using the proposed model is presented in Tables 1, 2, and 3 for the Czech Republic, Germany, and Denmark, respectively. The theoretical potential of BESS installation and use in electricity price peak was found to be around 9 - 16 % of cost-saving on average compared with the daily operation of PSA unit when the off-peak average electricity price was from 95 to 91 % of the daily average electricity price respectively. The practical potential of BESS installation will be significantly affected by BESS investment cost. For comparison, the LOX supply in the electricity peak was also analyzed. The theoretical potential of LOX supply for the same conditions was found slightly lower, around 8 - 11 % of cost-saving on average compared with the daily operation of the PSA unit. The values of cost-saving percentages for each country were plotted on the share of renewable energy sources (RES) in the energy mix for each country (Figure 2). The data confirm the effect of RES share on energy mix on the potential of electric energy time-shift. The analysis was executed for the static charging period from 0 a.m. to 6 a.m regardless of the actual electricity price. The system operated with a dynamically changed charging period based on the forecast prediction model for electricity price may further maximize the cost-saving. Table 1: On-site PSA unit with Battery Energy Storage System (BESS) – Czech Republic Description Unit January 8 April 8 July 8 October 7 Input data Daily average day-ahead price *1 EUR MWh-1 44.84 24.54 42.59 38.00 Average electricity price for charging period *1 EUR MWh-1 35.05 21.11 33.93 25.62 Off-peak average electricity price *1 EUR MWh-1 43.79 22.25 40.84 35.41 LOX price based on daily average el. price*1,2 EUR tO2-1 28.61 15.66 27.17 24.24 Single compression *3,4 PSA unit: cost for daily average el. price EUR d-1 866 474 823 734 PSA unit: cost for off-peak production EUR d-1 670 340 625 542 PSA unit: cost for peak production with BESS EUR d-1 141 85 137 103 Oxygen cost using BESS EUR d-1 811 425 761 645 Gain (+)/loss (-)*5 % 6.4 10.3 7.5 12.2 LOX consumed during electricity price peaks EUR d-1 143 78 136 121 Oxygen cost by combined PSA+LOX EUR d-1 813 419 761 663 Gain (+)/loss (-)*5 % 6.2 11.7 7.6 9.7 Dual compression *3,4 PSA unit: cost for daily average el. price EUR d-1 784 429 744 664 PSA unit: cost for off-peak production EUR d-1 606 308 565 490 PSA unit: cost for peak production with BESS EUR d-1 128 77 124 93 Oxygen cost using BESS EUR d-1 733 385 688 583 Gain (+)/loss (-)*5 % 6.4 10.3 7.5 12.2 LOX consumed during electricity price peaks EUR d-1 143 78 136 121 Oxygen cost by combined PSA+LOX EUR d-1 749 386 701 611 Gain (+)/loss (-)*5 % 4.4 9.9 5.8 8.0 Note:*1 Operation time = 24 h d-1, charging period from 0 a.m. to 6 a.m., peak period: from 8 a.m. to 10 a.m. and from 7 p.m. to 10 p.m. Note:*2 Specific electricity demand for LOX production: 0.638 MWh tO2-1. Note:*3 PSA unit: ambient air: temperature of 20 °C, pressure of 100 kPa (a), relative humidity of 70 %; compressed air; outlet temperature of 30 °C, outlet pressure of 750 kPa (a); specific electricity demand: single compression 0.805 MWh tO2-1, dual compression 0.728 MWh tO2-1. Note: *4: Calculation was performed for the specific oxygen production capacity of 1 t h-1. Note: *5 Gain/loss: related to the cost of PSA production for daily average electricity price. 472 Table 2: On-site PSA unit with Battery Energy Storage System (BESS) – Germany Description Unit January 8 April 8 July 8 October 7 Input data Daily average day-ahead price *1 EUR MWh-1 33.31 26.01 40.99 36.82 Average electricity price for charging period *1 EUR MWh-1 8.23 20.38 33.19 26.74 Off-peak average electricity price *1 EUR MWh-1 30.60 23.30 39.08 34.67 LOX price based on daily average el. price*1,2 EUR tO2-1 21.25 16.59 26.15 23.49 Single compression*3,4 PSA unit: cost for daily average el. price EUR d-1 644 502 792 711 PSA unit: cost for off-peak production EUR d-1 468 356 598 530 PSA unit: cost for peak production with BESS EUR d-1 33 82 134 108 Oxygen cost using BESS EUR d-1 501 438 731 638 Gain (+)/loss (-)*5 % 22.1 12.7 7.6 10.3 LOX consumed during electricity price peaks EUR d-1 106 83 131 117 Oxygen cost by combined PSA+LOX EUR d-1 574 439 729 648 Gain (+)/loss (-)*5 % 10.8 12.6 8.0 8.9 Dual compression *3,4 PSA unit: cost for daily average el. price EUR d-1 582 454 716 643 PSA unit: cost for off-peak production EUR d-1 423 322 541 480 PSA unit: cost for peak production with BESS EUR d-1 30 74 121 97 Oxygen cost using BESS EUR d-1 453 397 661 577 Gain (+)/loss (-)*5 % 22.1 12.7 7.6 10.3 LOX consumed during electricity price peaks EUR d-1 106 83 131 117 Oxygen cost by combined PSA+LOX EUR d-1 529 405 671 597 Gain (+)/loss (-)*5 % 9.0 10.8 6.3 7.2 Note:*1-5 see Table 1 in detail. Table 3: On-site PSA unit with Battery Energy Storage System (BESS) – Denmark Description Unit January 8 April 8 July 8 October 7 Input data Daily average day-ahead price *1 EUR MWh-1 24.44 25.72 38.17 32.50 Average electricity price for charging period *1 EUR MWh-1 8.06 20.30 22.70 11.61 Off-peak average electricity price *1 EUR MWh-1 22.60 23.11 35.52 29.22 LOX price based on daily average el. price*1,2 EUR tO2-1 15.59 16.41 24.35 20.73 Single compression *3,4 PSA unit: cost for daily average el. price EUR d-1 472 497 737 628 PSA unit: cost for off-peak production EUR d-1 346 353 543 447 PSA unit: cost for peak production with BESS EUR d-1 32 82 91 47 Oxygen cost using BESS EUR d-1 378 435 635 494 Gain (+)/loss (-)*5 % 19.9 12.4 13.9 21.4 LOX consumed during electricity price peaks EUR d-1 78 82 122 104 Oxygen cost by combined PSA+LOX EUR d-1 424 436 665 551 Gain (+)/loss (-)*5 % 10.3 12.3 9.8 12.3 Dual compression *3,4 PSA unit: cost for daily average el. price EUR d-1 427 449 667 568 PSA unit: cost for off-peak production EUR d-1 313 320 491 404 PSA unit: cost for peak production with BESS EUR d-1 29 74 83 42 Oxygen cost using BESS EUR d-1 342 394 574 446 Gain (+)/loss (-)*5 % 19.9 12.4 13.9 21.4 LOX consumed during electricity price peaks EUR d-1 78 82 122 104 Oxygen cost by combined PSA+LOX EUR d-1 391 402 613 508 Gain (+)/loss (-)*5 % 8.5 10.6 8.1 10.6 Note:*1-5 see Table 1 in detail. 473 Figure 2: Effect of the ratio of renewable energy sources (RES share) on cost-saving percentage: a) single compression (on the left), b) double compression (on the right) 4. Conclusions This paper aims to analyze the potential of a PSA unit connected to the battery energy storage system for more effective on-site oxygen production. The analysis was carried out for the Czech Republic, Germany, and Denmark. These countries differ significantly in the energy mix. The effect of intra-day electricity price fluctuations and energy storage system costs will be taken into account during the economic analysis. The theoretical potential of BESS installation and use in electricity price peak was found to be around 9 - 16 % of cost-saving on average compared with the daily operation of PSA unit when the off-peak average electricity price was from 95 to 91 % of the daily average electricity price respectively. Widening the price gap due to increasing RES share, the potential is growing. For comparison, the LOX supply in the electricity peak was also analyzed. The theoretical potential of LOX supply for the same conditions was found slightly lower, around 8 - 11 % of cost-saving on average compared with the daily operation of the PSA unit. Acknowledgements This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic under OP RDE grant number CZ.02.1.01/0.0/0.0/16_019/0000753 "Research center for low-carbon energy technologies". References Cao Y., Swartz Ch.L.E., Flores-Cerrillo J., 2017, Preemptive dynamic operation of cryogenic air separation units, AIChE J, 63, 3845-3859. Caspari A., Offermanns Ch., Schäfer P., Mhamdi A., Mitsos, A., 2019a, A flexible air separation process: 1. Design and steady‐state optimizations, AIChE J, 65, e16705. Caspari A., Offermanns Ch., Schäfer P., Mhamdi A., Mitsos A., 2019b, A flexible air separation process: 2. Optimal operation using economic model predictive control, AIChE J, 65, e16721. 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