- Email: [email protected]
Lithium-ion degradation at varying discharge rates
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
Sc i enc eDi r ect Sc i enc eDi r ect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 194–198 EnergyProcedia Procedia151 00 (2017) www.elsevier.com/locate/procedia
3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC, 3rd Annual Conference in Energy Storage andSheffield, Its Applications, 11–12 September 2018, UK 3rd CDT-ESA-AC, 11–12 September 2018, Sheffield, UK
Lithium-ion degradation at varying discharge rates The 15th International Symposium on District Heating and Cooling Lithium-ion degradation at varying discharge rates
Thomas S. Brydenaa *, Alexander Hollandaa, George Hiltonaa, Borislav Dimitrovaa, Carlos a a Thomas S. Bryden the *,Ponce Alexander Holland , George Hilton , Borislav Dimitrov , Carlos Assessing feasibility of using the heat demand-outdoor de León Albarrán , Andrew Cruden Ponce de León Albarrána, Andrew Crudena
temperature function for a long-term district heatSO14demand forecast Department of Engineering and the Environment, University of Southampton, Southampton 1BJ, United Kingdom a
Department of Engineering and the Environment, University of Southampton, Southampton SO14 1BJ, United Kingdom
a
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Using lithium-ion energy storage buffer energy between291 theAvenue electricity grid Daniel, and a load can enable higher power loads to operate VeoliatoRecherche & Innovation, Dreyfous 78520 Limay, France c Using lithium-ion energy storage bufferpotentially energy between the costly electricity grid and a load can enable44300 higher power loadssystems, to operate on lower power grid connections, thereby avoiding infrastructure upgrades. ForFrance such an Département Systèmesto Énergétiques et Environnement - IMT electricity Atlantique, 4grid rue Alfred Kastler, Nantes, on lower power connections, thereby potentially avoiding electricity grid infrastructure upgrades. battery For such systems, an estimation of thegrid lifetime of the lithium-ion battery is critical forcostly calculating the system costs. The lithium-ion may however estimation of the lifetime of the lithium-ion is critical calculating the system costs. The lithium-ion may however operate at elevated discharge rates where thebattery lifetime cannot for be easily estimated from datasheets. In this paperbattery lithium-ion lifetime operate at elevatedrates discharge rates where the lifetime cannot easily estimated datasheets. paper lithium-ion lifetime at two discharge is investigated experimentally. Two be lithium-ion cells arefrom cycled, one at In thethis manufacturer recommended Abstract at two discharge is investigated experimentally. are cycled, one atisthe manufacturer recommended discharge rate of 1rates C and one at the elevated dischargeTwo ratelithium-ion of 3 C. Thecells capacity degradation monitored and electrochemical discharge rate of 1 C and are oneperiodically at the elevated discharge of 3 cell C. The capacity degradation is monitored impedance measurements taken to track rate internal resistance growth. It is found that after and 400 electrochemical cycles the cells District heating areperiodically commonly addressed in internal the as one ofgrowth. the maximum most solutions forcycles decreasing the impedance measurements to % track resistance It iseffective found that after 400 the cells discharged at 1 C networks and 3 Care have capacities taken of 83.9 and 81.4literature %cell respectively the capacity recorded. Although this greenhouse gas emissions frombe the building sector. systems require investments which are being returned through heat discharged 1 Clarge, and it 3 should C have capacities ofover 83.9 % These and % respectively of passed the maximum capacity recorded. Although this difference isatnot noted that these 40081.4 cycles, 3260 A hhigh had through the cell cycled at 1 Cthe while sales. Due to the changed climate conditions and renovation demand in the future could difference is not large, itAshould bepassed noted that over these 400 cycles, 3260 had passed through cell being cycled atimpedance 1 decrease, C while 10 % less charge (2930 h) had though the cellbuilding being cycled at 3AC.hpolicies, Over the heat same cyclesthe the electrochemical prolonging the period. 10 % less charge (2930 A h)return though theatcell 3 C. Overby the34.7 same the%electrochemical results show the investment resistances ofhad thepassed cells discharged 1 Cbeing and 3cycled C haveatincreased % cycles and 57.4 respectively. impedance The main of this paper assess the feasibility using heat demandby – outdoor temperature function for heat demand results showscope the resistances of is thetocells discharged at 1 Cofand 3 Cthe have increased 34.7 % and 57.4 % respectively. forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Copyright © 2018 Elsevier Ltd. All rights reserved. Copyright © 2018 Elsevier Ltd. Allrights rightsreserved. reserved. buildingsand that vary in both construction period and3rd typology. weather scenarios (low,and medium, high) and three district Copyright © 2018 Elsevier Ltd. All Selection peer-review under responsibility of the Annual Three Conference in Energy Storage Its Applications, Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, renovation scenarios wereunder developed (shallow, intermediate, deep). To estimate the Storage error, obtained heat demand values were Selection and peer-review responsibility of the 3rd Annual Conference in Energy and Its Applications, 3rd CDT-ESA-AC. 3rd CDT-ESA-AC. compared with results from a dynamic heat demand model, previously developed and validated by the authors. 3rd CDT-ESA-AC. The results showed that when only weather change is considered, Keywords: Degradation, Electric vehicles, Fast charging, Lithium-ion batterythe margin of error could be acceptable for some applications (the errorDegradation, in annual demand was lower 20%Lithium-ion for all weather Keywords: Electric vehicles, Fast than charging, batteryscenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction renovation scenarios considered). the are otherfacing hand, increasing function intercept increased per quantities decade (depending on the Electricity grids around the On world pressures fromfor the7.8-12.7% increasing of electricity coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and Electricity grids around the world are facing increasing pressures from the increasing quantities of electricity produced by intermittent renewable sources and from the increasing number of electric vehicles requiring charging. improve the accuracy of heat demand estimations. produced by intermittent renewable sources and from the increasing number of electric vehicles requiring charging. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author.
E-mail address:author. [email protected] * Corresponding Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 10.1016/j.egypro.2018.09.047
2
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
195
Renewable electricity production and consumption have increased four times in the last ten years, from around 100 Mtoe in 2006 to around 400 Mtoe in 2016 [1]. Globally, in 2016 there were 2 million electric vehicles on the road, with the International Energy Agency predicting this will increase to between 40-70 million in 2025 [2]. In traditional electricity grids electricity must be generated at the same rate that it is demanded and as such electricity generation is varied depending on demand. In the future this will be harder as electric vehicles will increase electricity demand at different hours of the day and renewable electricity generation is intermittent, not controllable. The use of energy storage on the electricity grid can alleviate these pressures as the electricity demand is decoupled from the electricity generation. Thus one application of energy storage on the grid is to act as a buffer between the grid and load [3]. The energy storage can be charged at a low rate from the grid and then, when the load requires power, the energy storage discharges at a high rate. The energy storage therefore allows high power to be transferred to the load from a low power grid connection. This can be useful at locations where the grid connection is limited and where it may be expensive to upgrade the grid infrastructure. Examples of potential uses include electric vehicle fast charging stations where electricity demand will be high but not continuous throughout the day [4]. Energy storage solutions vary from small devices storing small quantities of energy for seconds through to large devices storing large quantities of energy storing energy for many hours or days. The choice of energy storage devices depends on the application. For the case of an energy storage buffer the energy quantity is likely to be small to medium for a period of seconds to one hour, lithium ion batteries are therefore an appropriate storage solution [5]. When designing an energy storage buffer system the costs must be considered, including how often the system must be replaced. The lifetime of lithium ion batteries is therefore critical information. For a lithium ion buffering system the lifetime of the lithium ion batteries may not be able to be taken directly from the datasheet as the lithium ion batteries may have to operate at elevated discharge rates, here defined as high rate operation, which is defined as rates over the manufacturer recommended rate. High rate operation is a known cause of lithium ion cell degradation, other causes include, temperature, operation of extended voltage levels, state of charge and depth of discharge [6]. It is therefore important to understand how the degradation of lithium ion batteries varies with discharge rate. In this paper, the effect of elevated discharge rates on the lifetime of lithium ion cells is investigated. Experiments are conducted where lithium ion cells are cycled at different rates and the degradation is monitored using a combination of techniques, including Electrochemical Impedance Spectroscopy (EIS). In Section 2 the experimental method is described, in Section 3 the results are presented, and in Section 4 the conclusions are summarized. 2. Method Two lithium ion cells with a cathode chemistry of Nickel Manganese Cobalt (NMC) are cycled, one at the standard manufacturer recommended discharge rate (1 C) [7] and one at an elevated discharge rate (3 C) using the constant current method. The 3 C discharge rate was chosen based on temperature considerations, at the chosen rate of 3 C the maximum surface temperature reached is around 60 °C. At higher discharge rates the temperature exceeds 60 °C and so it was considered not safe to cycle the cell at higher rates. Both cells are charged using the constant current method at the manufacturer standard rate (0.5 C). The cells are cycled between the manufacturer suggested voltages of 2.75 V and 4.2 V, with one cycle being defined as a charge and discharge between these voltages. Details of the cell are given in Table 1, each cell is new at the start of testing. To be able to compare capacity degradation, the capacity of the cells must be measured at the same discharge rate, hence periodically the cell being discharged at 3 C is discharged at 1 C for comparison. The period chosen is every 40 cycles between 0 and 200 cycles and every 100 cycles after. In addition to measuring the capacity degradation EIS is conducted periodically to evaluate how the cell resistances varies with cycles. The EIS is conducted by applying a voltage change of 10 mV between the frequencies of 10 kHz to 0.01 Hz. To compare cell resistances equivalent circuit fitting was used, the circuit chosen for comparison is seen in Fig. 1, taken from a Battery Systems Engineering Workshop run by Imperial College London.
Fig. 1. Equivalent circuit, L1 is inductance, R1 is the series resistance, C1 is the double layer capacitance, R2 is the solid electrolyte interphase (SEI) resistance, CPE1 is the constant phase element, R3 is the charge transfer resistance and Ws1 is the Warburg coefficient.
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
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Table 1. Details of the Cylindrical 26650 Lithium-ion Nickel Manganese Cobalt cell. Name
AAPortable Power INR-26650-5000
Nominal voltage (V)
3.6
Capacity (A h)
5
Mass (g)
95
Standard / maximum charge current (A)
2.5 / 5
Standard / maximum discharge current (A)
5 / 15
Thermocouples are placed on the cell surfaces for safety and to measure the cell temperatures, a second thermocouple measured the ambient temperature. The cells are cycled using a Maccor 4200 battery analyser, the EIS conducted using a Solartron 1400/1470E and the equivalent circuit fitting conducted using the ZView software. A good connection with low resistance between the cell and the battery analyser is important to ensure accurate voltage measurements and to avoid additional heat generation due to poor electrical connections. A good connection between the EIS machine is even more important as poor connections can greatly affect the accuracy of results. To ensure a good connection wires are soldered to each cell and a four wire setup, two current carrying wires and two voltage sensing wires, are used. The cells are cycled once before testing and EIS measurements were taken before the cycling was started, which showed that the capacities (4.20 A h and 4.26 A h) and resistances (0.0219 Ω and 0.0216 Ω) of both cells are similar. The connections were also assumed to be good as EIS measurements were taken three times, disconnecting all connections between each test, and the results showed the same resistances for each test. 3. Results The cells are each cycled 400 times, with capacity results and EIS results taken after 0, 40, 80, 120, 160, 200, 300 and 400 cycles. Single cycle voltage and temperature results are shown in Fig. 2. It is clear from Fig. 2b that the cell that is discharged at 3 C gets significantly hotter than the cell discharged at 1C. The capacity values of around 4.4 A h are lower than the 5 A h rated capacity of the cell because only constant current charging is conducted to save time, to get the whole capacity the cell would need to be charged using the constant current constant voltage technique. 3.1. Capacity Results The capacity of the cells versus the number of cycles can be seen in Fig. 3, both the capacity (A h) and energy capacity (W h) are plotted. The capacity results have been plotted for every cycle for the cell discharged at 1 C, while for the cell discharged at 3 C only the comparable results when the cell is discharged at 1 C are plotted. After 400 cycles the cell discharged at 3 C had a capacity of 3.63 A h while the cell discharged at 1 C had a capacity of 3.74 A h. These values are 81.4 % and 83.9 % respectively of the maximum capacity recorded on cycle 7 of 4.46 A h. At 80 % capacity cells are often considered at the end of their life and so these cells are close the end of their life.
Fig. 2. Initial cycle plots (a) Voltage during charge and discharge (b) Temperatures during discharge.
4
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
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Fig. 3. Degradation with cycles (a) Capacity (b) Energy.
It can also be seen that the cell discharged at 3 C does lose capacity faster than the cell discharged at 1 C. Between 40 and 200 cycles the capacities diverge, with both cells having a capacity of about 4.37 A h at 40 cycles. After 200 cycles the cell cycled at 3 C has a capacity 0.14 A h lower than the cell cycled at 1 C, 3.92 A h compared to 4.06 A h. However between 200 and 400 cycles this difference stays roughly the same and in fact decreases slightly to 0.11 A h, 3.63 A h compared to 3.74 A h. It should be noted that, because of the degradation, the cell being discharged at 3 C has less charge throughput per cycle than the cell being cycled at 1 C. After the 400 cycles 3260 A h had passed through the cell being cycled at 1 C while 10 % less charge (2930 A h) had passed though the cell being cycled at 3 C. 3.2. Electrochemical Impedance Spectroscopy Results The full EIS results obtained are shown in Fig. 4. It is clear that for both cells the curves are gradually shifting to the right and becoming wider with increased cycles, indicating resistance growth. When comparing the cells, the cell being discharged at 3 C is shifting to the right and becoming wider earlier than the cell being discharged at 1 C, indicating the resistance of the cell discharged at 3 C is increasing faster than the cell being discharged at 1 C. The equivalent circuit fitting results are then obtained, the obtained resistance values can be seen in Fig. 5. As can be seen in Fig. 5d the sum of all resistances increases with cycles and the resistance of the cell discharged at 3 C increases faster than the cell discharged at 1 C. The overall resistance of the cell discharged at 1 C has increased 34.7 % (from 21.9 mΩ to 29.5 mΩ) between 0 and 400 cycles, while the cell discharged at 3C has increased 57.4 % (from 21.6 mΩ to 34.0 mΩ) over the same period.
Fig. 4. Impedance results (a) Cell cycled at 1 C (b) Cell cycled at 3 C.
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
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Fig. 5. Equivalent circuit fitting resistance results (a) Series (b) SEI (c) Charge transfer (d) Sum of all resistances.
4. Conclusions The cell cycled at a higher rate of 3 C degraded faster than the cell cycled at a standard rate of 1 C. After 400 cycles the cell discharged at 3 C had 81.4 % of the maximum capacity measured while the cell discharged at 1 C had 83.9 % of the maximum capacity. The EIS results show agreement as the resistance growth is clearly greater in the cell cycled at higher rates. Using equivalent circuit modelling, the resistance growth has been quantified, after 400 cycles overall resistance of the cell discharged at 1 C had grown by 34.7 %, while the cell discharged at 3 C grew 57.4 %. It is hard to draw conclusions from the individual resistances in Fig. 5 and future work could use different equivalent circuits. Although the cell that is discharged at a higher rate degrades faster it is not possible from this study to determine whether the accelerated degradation is caused by the higher rate or by the higher temperature. Cycling rate and temperature are both known to accelerate degradation in lithium ion batteries. Future work could use cooling of the cells to keep the cells at the same temperature, thereby ensuring any accelerated degradation was caused by the cycling rate. This however is likely hard as at high rates cells heat up significantly and keeping the cell cooler will increase the resistance meaning more heat will be generated in the cell. The results from this paper show how lithium ion cells discharged at higher rates degrade faster than cells cycled at standard rates. This must be considered when designing lithium ion energy storage solutions for the electricity grid infrastructure. When performing initial engineering studies it must be considered that, if high rates are required from the energy storage, the energy storage will have to be replaced more frequently, which in turn adds to the system cost. Acknowledgements This research was supported through two grants “EPSRC Centre for Doctoral Training in Energy Storage and its Applications” EP/L016818/1 and “ELEVATE (ELEctrochemical Vehicle Advanced TEchnology)” EP/M009394/1. References [1] BP. “BP Statistical Review of World Energy”, (2017) BP p.l.c. [2] IEA. “Global EV Outlook 2017 Two million and counting”, (2017) International Energy Agency [3] Akhil, Abbas A, Huff, Georgianne, Currier, Aileen B, Kaun, Benjamin C, Rastler, Dan M, Chen, Stella Bingqing, et al. “DOE/EPRI Electricity Storage Handbook in Collaboration with NRECA”, (2015) Albuquerque, Sandia National Laboratories [4] Bryden, Thomas, Hilton, George, Cruden, Andrew and Holton, Tim. "Electric vehicle fast charging station usage and power requirements" Energy 152 (2018): 322-332, doi: 10.1016/j.energy.2018.03.149 [5] Bryden, Thomas S, Cruden, Andrew J, Hilton, George, Dimitrov, Borislav H, Ponce de León, Carlos and Mortimer, Alan “Off-vehicle energy store selection for high rate EV charging station” 6th Hybrid and Electric Vehicles Conference, (2016), doi: 10.1049/cp.2016.0986 [6] Bryden, Thomas, Dimitrov, Borislav, Hilton, George, Ponce de León, Carlos, Bugryniec, Peter, Brown, Solomon, et al. “Methodology to determine the heat capacity of lithium-ion cells" Journal of Power Sources 395 (2018): 369-378, doi: 10.1016/j.jpowsour.2018.05.084 [7] AA Portable Power Corp. “INR-26650-5000 Specification” (2018) Available: http://www.batteryspace.com/prod-specs/9869.pdf
Sc i enc eDi r ect Sc i enc eDi r ect
Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2018) 000–000 194–198 EnergyProcedia Procedia151 00 (2017) www.elsevier.com/locate/procedia
3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC, 3rd Annual Conference in Energy Storage andSheffield, Its Applications, 11–12 September 2018, UK 3rd CDT-ESA-AC, 11–12 September 2018, Sheffield, UK
Lithium-ion degradation at varying discharge rates The 15th International Symposium on District Heating and Cooling Lithium-ion degradation at varying discharge rates
Thomas S. Brydenaa *, Alexander Hollandaa, George Hiltonaa, Borislav Dimitrovaa, Carlos a a Thomas S. Bryden the *,Ponce Alexander Holland , George Hilton , Borislav Dimitrov , Carlos Assessing feasibility of using the heat demand-outdoor de León Albarrán , Andrew Cruden Ponce de León Albarrána, Andrew Crudena
temperature function for a long-term district heatSO14demand forecast Department of Engineering and the Environment, University of Southampton, Southampton 1BJ, United Kingdom a
Department of Engineering and the Environment, University of Southampton, Southampton SO14 1BJ, United Kingdom
a
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc
Abstract Abstract a IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Using lithium-ion energy storage buffer energy between291 theAvenue electricity grid Daniel, and a load can enable higher power loads to operate VeoliatoRecherche & Innovation, Dreyfous 78520 Limay, France c Using lithium-ion energy storage bufferpotentially energy between the costly electricity grid and a load can enable44300 higher power loadssystems, to operate on lower power grid connections, thereby avoiding infrastructure upgrades. ForFrance such an Département Systèmesto Énergétiques et Environnement - IMT electricity Atlantique, 4grid rue Alfred Kastler, Nantes, on lower power connections, thereby potentially avoiding electricity grid infrastructure upgrades. battery For such systems, an estimation of thegrid lifetime of the lithium-ion battery is critical forcostly calculating the system costs. The lithium-ion may however estimation of the lifetime of the lithium-ion is critical calculating the system costs. The lithium-ion may however operate at elevated discharge rates where thebattery lifetime cannot for be easily estimated from datasheets. In this paperbattery lithium-ion lifetime operate at elevatedrates discharge rates where the lifetime cannot easily estimated datasheets. paper lithium-ion lifetime at two discharge is investigated experimentally. Two be lithium-ion cells arefrom cycled, one at In thethis manufacturer recommended Abstract at two discharge is investigated experimentally. are cycled, one atisthe manufacturer recommended discharge rate of 1rates C and one at the elevated dischargeTwo ratelithium-ion of 3 C. Thecells capacity degradation monitored and electrochemical discharge rate of 1 C and are oneperiodically at the elevated discharge of 3 cell C. The capacity degradation is monitored impedance measurements taken to track rate internal resistance growth. It is found that after and 400 electrochemical cycles the cells District heating areperiodically commonly addressed in internal the as one ofgrowth. the maximum most solutions forcycles decreasing the impedance measurements to % track resistance It iseffective found that after 400 the cells discharged at 1 C networks and 3 Care have capacities taken of 83.9 and 81.4literature %cell respectively the capacity recorded. Although this greenhouse gas emissions frombe the building sector. systems require investments which are being returned through heat discharged 1 Clarge, and it 3 should C have capacities ofover 83.9 % These and % respectively of passed the maximum capacity recorded. Although this difference isatnot noted that these 40081.4 cycles, 3260 A hhigh had through the cell cycled at 1 Cthe while sales. Due to the changed climate conditions and renovation demand in the future could difference is not large, itAshould bepassed noted that over these 400 cycles, 3260 had passed through cell being cycled atimpedance 1 decrease, C while 10 % less charge (2930 h) had though the cellbuilding being cycled at 3AC.hpolicies, Over the heat same cyclesthe the electrochemical prolonging the period. 10 % less charge (2930 A h)return though theatcell 3 C. Overby the34.7 same the%electrochemical results show the investment resistances ofhad thepassed cells discharged 1 Cbeing and 3cycled C haveatincreased % cycles and 57.4 respectively. impedance The main of this paper assess the feasibility using heat demandby – outdoor temperature function for heat demand results showscope the resistances of is thetocells discharged at 1 Cofand 3 Cthe have increased 34.7 % and 57.4 % respectively. forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 Copyright © 2018 Elsevier Ltd. All rights reserved. Copyright © 2018 Elsevier Ltd. Allrights rightsreserved. reserved. buildingsand that vary in both construction period and3rd typology. weather scenarios (low,and medium, high) and three district Copyright © 2018 Elsevier Ltd. All Selection peer-review under responsibility of the Annual Three Conference in Energy Storage Its Applications, Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, renovation scenarios wereunder developed (shallow, intermediate, deep). To estimate the Storage error, obtained heat demand values were Selection and peer-review responsibility of the 3rd Annual Conference in Energy and Its Applications, 3rd CDT-ESA-AC. 3rd CDT-ESA-AC. compared with results from a dynamic heat demand model, previously developed and validated by the authors. 3rd CDT-ESA-AC. The results showed that when only weather change is considered, Keywords: Degradation, Electric vehicles, Fast charging, Lithium-ion batterythe margin of error could be acceptable for some applications (the errorDegradation, in annual demand was lower 20%Lithium-ion for all weather Keywords: Electric vehicles, Fast than charging, batteryscenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction renovation scenarios considered). the are otherfacing hand, increasing function intercept increased per quantities decade (depending on the Electricity grids around the On world pressures fromfor the7.8-12.7% increasing of electricity coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and Electricity grids around the world are facing increasing pressures from the increasing quantities of electricity produced by intermittent renewable sources and from the increasing number of electric vehicles requiring charging. improve the accuracy of heat demand estimations. produced by intermittent renewable sources and from the increasing number of electric vehicles requiring charging. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author.
E-mail address:author. [email protected] * Corresponding Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 3rd Annual Conference in Energy Storage and Its Applications, 3rd CDT-ESA-AC. 10.1016/j.egypro.2018.09.047
2
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
195
Renewable electricity production and consumption have increased four times in the last ten years, from around 100 Mtoe in 2006 to around 400 Mtoe in 2016 [1]. Globally, in 2016 there were 2 million electric vehicles on the road, with the International Energy Agency predicting this will increase to between 40-70 million in 2025 [2]. In traditional electricity grids electricity must be generated at the same rate that it is demanded and as such electricity generation is varied depending on demand. In the future this will be harder as electric vehicles will increase electricity demand at different hours of the day and renewable electricity generation is intermittent, not controllable. The use of energy storage on the electricity grid can alleviate these pressures as the electricity demand is decoupled from the electricity generation. Thus one application of energy storage on the grid is to act as a buffer between the grid and load [3]. The energy storage can be charged at a low rate from the grid and then, when the load requires power, the energy storage discharges at a high rate. The energy storage therefore allows high power to be transferred to the load from a low power grid connection. This can be useful at locations where the grid connection is limited and where it may be expensive to upgrade the grid infrastructure. Examples of potential uses include electric vehicle fast charging stations where electricity demand will be high but not continuous throughout the day [4]. Energy storage solutions vary from small devices storing small quantities of energy for seconds through to large devices storing large quantities of energy storing energy for many hours or days. The choice of energy storage devices depends on the application. For the case of an energy storage buffer the energy quantity is likely to be small to medium for a period of seconds to one hour, lithium ion batteries are therefore an appropriate storage solution [5]. When designing an energy storage buffer system the costs must be considered, including how often the system must be replaced. The lifetime of lithium ion batteries is therefore critical information. For a lithium ion buffering system the lifetime of the lithium ion batteries may not be able to be taken directly from the datasheet as the lithium ion batteries may have to operate at elevated discharge rates, here defined as high rate operation, which is defined as rates over the manufacturer recommended rate. High rate operation is a known cause of lithium ion cell degradation, other causes include, temperature, operation of extended voltage levels, state of charge and depth of discharge [6]. It is therefore important to understand how the degradation of lithium ion batteries varies with discharge rate. In this paper, the effect of elevated discharge rates on the lifetime of lithium ion cells is investigated. Experiments are conducted where lithium ion cells are cycled at different rates and the degradation is monitored using a combination of techniques, including Electrochemical Impedance Spectroscopy (EIS). In Section 2 the experimental method is described, in Section 3 the results are presented, and in Section 4 the conclusions are summarized. 2. Method Two lithium ion cells with a cathode chemistry of Nickel Manganese Cobalt (NMC) are cycled, one at the standard manufacturer recommended discharge rate (1 C) [7] and one at an elevated discharge rate (3 C) using the constant current method. The 3 C discharge rate was chosen based on temperature considerations, at the chosen rate of 3 C the maximum surface temperature reached is around 60 °C. At higher discharge rates the temperature exceeds 60 °C and so it was considered not safe to cycle the cell at higher rates. Both cells are charged using the constant current method at the manufacturer standard rate (0.5 C). The cells are cycled between the manufacturer suggested voltages of 2.75 V and 4.2 V, with one cycle being defined as a charge and discharge between these voltages. Details of the cell are given in Table 1, each cell is new at the start of testing. To be able to compare capacity degradation, the capacity of the cells must be measured at the same discharge rate, hence periodically the cell being discharged at 3 C is discharged at 1 C for comparison. The period chosen is every 40 cycles between 0 and 200 cycles and every 100 cycles after. In addition to measuring the capacity degradation EIS is conducted periodically to evaluate how the cell resistances varies with cycles. The EIS is conducted by applying a voltage change of 10 mV between the frequencies of 10 kHz to 0.01 Hz. To compare cell resistances equivalent circuit fitting was used, the circuit chosen for comparison is seen in Fig. 1, taken from a Battery Systems Engineering Workshop run by Imperial College London.
Fig. 1. Equivalent circuit, L1 is inductance, R1 is the series resistance, C1 is the double layer capacitance, R2 is the solid electrolyte interphase (SEI) resistance, CPE1 is the constant phase element, R3 is the charge transfer resistance and Ws1 is the Warburg coefficient.
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
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Table 1. Details of the Cylindrical 26650 Lithium-ion Nickel Manganese Cobalt cell. Name
AAPortable Power INR-26650-5000
Nominal voltage (V)
3.6
Capacity (A h)
5
Mass (g)
95
Standard / maximum charge current (A)
2.5 / 5
Standard / maximum discharge current (A)
5 / 15
Thermocouples are placed on the cell surfaces for safety and to measure the cell temperatures, a second thermocouple measured the ambient temperature. The cells are cycled using a Maccor 4200 battery analyser, the EIS conducted using a Solartron 1400/1470E and the equivalent circuit fitting conducted using the ZView software. A good connection with low resistance between the cell and the battery analyser is important to ensure accurate voltage measurements and to avoid additional heat generation due to poor electrical connections. A good connection between the EIS machine is even more important as poor connections can greatly affect the accuracy of results. To ensure a good connection wires are soldered to each cell and a four wire setup, two current carrying wires and two voltage sensing wires, are used. The cells are cycled once before testing and EIS measurements were taken before the cycling was started, which showed that the capacities (4.20 A h and 4.26 A h) and resistances (0.0219 Ω and 0.0216 Ω) of both cells are similar. The connections were also assumed to be good as EIS measurements were taken three times, disconnecting all connections between each test, and the results showed the same resistances for each test. 3. Results The cells are each cycled 400 times, with capacity results and EIS results taken after 0, 40, 80, 120, 160, 200, 300 and 400 cycles. Single cycle voltage and temperature results are shown in Fig. 2. It is clear from Fig. 2b that the cell that is discharged at 3 C gets significantly hotter than the cell discharged at 1C. The capacity values of around 4.4 A h are lower than the 5 A h rated capacity of the cell because only constant current charging is conducted to save time, to get the whole capacity the cell would need to be charged using the constant current constant voltage technique. 3.1. Capacity Results The capacity of the cells versus the number of cycles can be seen in Fig. 3, both the capacity (A h) and energy capacity (W h) are plotted. The capacity results have been plotted for every cycle for the cell discharged at 1 C, while for the cell discharged at 3 C only the comparable results when the cell is discharged at 1 C are plotted. After 400 cycles the cell discharged at 3 C had a capacity of 3.63 A h while the cell discharged at 1 C had a capacity of 3.74 A h. These values are 81.4 % and 83.9 % respectively of the maximum capacity recorded on cycle 7 of 4.46 A h. At 80 % capacity cells are often considered at the end of their life and so these cells are close the end of their life.
Fig. 2. Initial cycle plots (a) Voltage during charge and discharge (b) Temperatures during discharge.
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Fig. 3. Degradation with cycles (a) Capacity (b) Energy.
It can also be seen that the cell discharged at 3 C does lose capacity faster than the cell discharged at 1 C. Between 40 and 200 cycles the capacities diverge, with both cells having a capacity of about 4.37 A h at 40 cycles. After 200 cycles the cell cycled at 3 C has a capacity 0.14 A h lower than the cell cycled at 1 C, 3.92 A h compared to 4.06 A h. However between 200 and 400 cycles this difference stays roughly the same and in fact decreases slightly to 0.11 A h, 3.63 A h compared to 3.74 A h. It should be noted that, because of the degradation, the cell being discharged at 3 C has less charge throughput per cycle than the cell being cycled at 1 C. After the 400 cycles 3260 A h had passed through the cell being cycled at 1 C while 10 % less charge (2930 A h) had passed though the cell being cycled at 3 C. 3.2. Electrochemical Impedance Spectroscopy Results The full EIS results obtained are shown in Fig. 4. It is clear that for both cells the curves are gradually shifting to the right and becoming wider with increased cycles, indicating resistance growth. When comparing the cells, the cell being discharged at 3 C is shifting to the right and becoming wider earlier than the cell being discharged at 1 C, indicating the resistance of the cell discharged at 3 C is increasing faster than the cell being discharged at 1 C. The equivalent circuit fitting results are then obtained, the obtained resistance values can be seen in Fig. 5. As can be seen in Fig. 5d the sum of all resistances increases with cycles and the resistance of the cell discharged at 3 C increases faster than the cell discharged at 1 C. The overall resistance of the cell discharged at 1 C has increased 34.7 % (from 21.9 mΩ to 29.5 mΩ) between 0 and 400 cycles, while the cell discharged at 3C has increased 57.4 % (from 21.6 mΩ to 34.0 mΩ) over the same period.
Fig. 4. Impedance results (a) Cell cycled at 1 C (b) Cell cycled at 3 C.
Thomas S. Bryden et al. / Energy Procedia 151 (2018) 194–198 Author name / Energy Procedia 00 (2018) 000–000
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5
Fig. 5. Equivalent circuit fitting resistance results (a) Series (b) SEI (c) Charge transfer (d) Sum of all resistances.
4. Conclusions The cell cycled at a higher rate of 3 C degraded faster than the cell cycled at a standard rate of 1 C. After 400 cycles the cell discharged at 3 C had 81.4 % of the maximum capacity measured while the cell discharged at 1 C had 83.9 % of the maximum capacity. The EIS results show agreement as the resistance growth is clearly greater in the cell cycled at higher rates. Using equivalent circuit modelling, the resistance growth has been quantified, after 400 cycles overall resistance of the cell discharged at 1 C had grown by 34.7 %, while the cell discharged at 3 C grew 57.4 %. It is hard to draw conclusions from the individual resistances in Fig. 5 and future work could use different equivalent circuits. Although the cell that is discharged at a higher rate degrades faster it is not possible from this study to determine whether the accelerated degradation is caused by the higher rate or by the higher temperature. Cycling rate and temperature are both known to accelerate degradation in lithium ion batteries. Future work could use cooling of the cells to keep the cells at the same temperature, thereby ensuring any accelerated degradation was caused by the cycling rate. This however is likely hard as at high rates cells heat up significantly and keeping the cell cooler will increase the resistance meaning more heat will be generated in the cell. The results from this paper show how lithium ion cells discharged at higher rates degrade faster than cells cycled at standard rates. This must be considered when designing lithium ion energy storage solutions for the electricity grid infrastructure. When performing initial engineering studies it must be considered that, if high rates are required from the energy storage, the energy storage will have to be replaced more frequently, which in turn adds to the system cost. Acknowledgements This research was supported through two grants “EPSRC Centre for Doctoral Training in Energy Storage and its Applications” EP/L016818/1 and “ELEVATE (ELEctrochemical Vehicle Advanced TEchnology)” EP/M009394/1. References [1] BP. “BP Statistical Review of World Energy”, (2017) BP p.l.c. [2] IEA. “Global EV Outlook 2017 Two million and counting”, (2017) International Energy Agency [3] Akhil, Abbas A, Huff, Georgianne, Currier, Aileen B, Kaun, Benjamin C, Rastler, Dan M, Chen, Stella Bingqing, et al. “DOE/EPRI Electricity Storage Handbook in Collaboration with NRECA”, (2015) Albuquerque, Sandia National Laboratories [4] Bryden, Thomas, Hilton, George, Cruden, Andrew and Holton, Tim. "Electric vehicle fast charging station usage and power requirements" Energy 152 (2018): 322-332, doi: 10.1016/j.energy.2018.03.149 [5] Bryden, Thomas S, Cruden, Andrew J, Hilton, George, Dimitrov, Borislav H, Ponce de León, Carlos and Mortimer, Alan “Off-vehicle energy store selection for high rate EV charging station” 6th Hybrid and Electric Vehicles Conference, (2016), doi: 10.1049/cp.2016.0986 [6] Bryden, Thomas, Dimitrov, Borislav, Hilton, George, Ponce de León, Carlos, Bugryniec, Peter, Brown, Solomon, et al. “Methodology to determine the heat capacity of lithium-ion cells" Journal of Power Sources 395 (2018): 369-378, doi: 10.1016/j.jpowsour.2018.05.084 [7] AA Portable Power Corp. “INR-26650-5000 Specification” (2018) Available: http://www.batteryspace.com/prod-specs/9869.pdf