RELIABILITY-BASED OPERATIONAL STRATEGIES FOR ENERGY STORAGE SYSTEMS IN TRASPORT SYSTEMS APPLICATIONS

Transcription

1 RELIABILITY-BASED OPERATIONAL STRATEGIES FOR ENERGY STORAGE SYSTEMS IN TRASPORT SYSTEMS APPLICATIONS Prof. Evangelos Dialynas Electric Energy Systems Laboratory, School of Electrical and Computer Engineering NaJonal Technical University of Athens - Greece 1

2 Introduc0on The BaJery Energy Storage Systems (BESS) have already been used in certain applica0ons. Their use in transporta0on systems prac0cally covers the daily opera0on of the electric vehicles (EV). The electric vehicles are not only considered as green transporta0on means, but they can also be envisaged as an ac0ve and, more importantly, flexible part of the future electricity distribu0on grid. Their bajeries can act as loads during valley periods and hours with high produc0on of Renewable Energy Sources (RES) in the neighborhood while they can also be electricity producers during peak hours. RES can also be used effec0vely in EV charging facili0es cons0tu0ng ajrac0ve green solu0ons in urban environment. This new role of BESS in transport systems applicajons requires an increased level of reliability performance as well as accurate real Jme informajon concerning their State of Charge (SoC) and State of Health (SoH). 2

3 Introduc0on The SoC is a variable of the bajery in terms of its current or power and its temperature. The exis0ng opera0onal strategies of the BESS do not consider very important parameters such as their: reliability indices - maintenance requirements cycle and calendar aging discharging pajern towards the electricity grid that has different characteris0cs from the respec0ve ones of normal EVs. The SoH of the BESS is ignored, which may lead to the wrong es0ma0on of their SoC and probably to addi0onal failures. Appropriate models and tools are described for achieving an op0mized opera0onal strategy of EVs. 3

4 BaJery Aging Model The available energy capacity of a bajery decreases over its service life0me due to a loss of charge capacity and a rise in internal resistance. Calendar and cycle aging are modeled as two independent addi0ve processes. SoH is the rate of the current usable capacity (with respect to 0me) Q b (t) and its nominal capacity Q bo. At the end of its life, it is normally assumed that a typical value of SoH is about 80%. It is assumed that the current annual degrada0on rate γ is the sum of two independent rates γ Τ and γ Ο that are func0ons of the internal stored energy (SoE) and bajery power P. Q b (t) = Q bo (1-γ) t 4

5 BaJery Calendar Aging Appropriate models of capacity loss have been proposed in literature for the es0ma0on of Q b (t) with respect to γ Τ and a reference temperature. A linear model, an exponen0al model, a parabolic model and a general α model have been used by making suitable approxima0ons. By taking into account the uncertain0es inherent in an aging model, the exponen0al model provides acceptable results with less than 2% error compared with the parabolic model and with less than 20% error in respect to the general α model. Typical values of life0me for bajeries at 80% SoC by using four different models 5

6 BaJery Cycle Aging Cycle aging only occurs when the bajery is used. It is natural to measure the energy capacity loss over the total cumulated energy charged or discharged from the bajery. A linear model for the instantaneous degrada0on rate δ p is assumed that can be used for the simula0on of exis0ng data of bajeries and the calcula0on of lost capacity P(t) over a 0me dura0on ΔΤ. The annual degrada0on rate γ p is calculated as the ra0o of P(t) and ΔΤ. 6

7 SoC Es0ma0on The accurate es0ma0on of SoC and the consequent response of the energy storage management system is cri0cal, especially during steep ramps of the demand. 7

8 SoC Es0ma0on The linear method of SoC es0ma0on is the most commonly used for these cases. Τhe DC-bus voltage and current are measured con0nuously. Τhe normalized measurements of the DC-bus are compared to the characteris0c curves and the new SoC is determined from the typical bajery curves, aher the proper 0me-delay and voltage-hysteresis has been considered. 8

9 Misleading SoC Es0ma0ons Tests have shown that the frequent switching (on-off) of large loads (steep rise of demand) can lead to big values of discharging currents, which in their turn, cons0tute misleading indica0ons for the correct es0ma0on of the SoC by using the linear method. The term pseudo State of Charge (pseudo-soc) is defined as the SoC that is assigned to the bajeries on purpose, when big values of discharging currents appear in a 2-minutes period and the (Vmean-2, Imean-2) point skips one of the SoC areas in the previous figure. 9

10 Op0miza0on Procedure Calendar and cycle aging must be integrated into the opera0onal strategy of the BESS together with the calcula0on of the capacity lost during the period of opera0on and its impact on the system cost. It is assumed that: o the energy capacity is equal to the charge capacity by neglec0ng the resistance increase. o SoE is treated as equivalent to SoC since the resistance is rela0vely constant over SoC. o the temperature dependence of calendar aging is ignored (bajery temperature varies slightly). An appropriate equa0on for evalua0ng the energy capacity loss has been developed. The cost of lost capacity depends on the nominal replacement cost of the bajery (future investment). 10

11 Op0miza0on Procedure The impact of aging can be expressed in terms of the Net Present Value (NPV) of the investment using a real interest rate r and an average annual degrada0on rate γ. An appropriate method (myopic approach) is used that represents the effect of marginal capacity loss on the NPV by assuming that the degrada0on rate γ remains constant over the op0miza0on period. Another method (average aging approach) is used by assuming that the op0miza0on period is typical of the bajery opera0on and the degrada0on rate is similar to that of the planning horizon (use of the deriva0ve of the 0me to replacement with respect to γ ). Typical case studies have shown that the minimal aging opera0onal strategy may differ from the op0mal schedule by maximizing only the opera0onal revenues. 11

12 Photovoltaic Systems in EV Charging Facili0es Applica0on of an economic Dispatch Op0miza0on algorithm by taking into account the capital expenditure as well as the aging impact of the bajeries. This also contributes to an increased reliability performance of the BESS and leads to different opera0onal pajerns. 12

13 Typical Daily Produc0on Schedule of Photovoltaic Systems 13

14 EinT2016 1st Interna0onal Conference ENERGY in TRANSPORTATION 2016 Photovoltaic Systems in EV Charging Facili0es Typical example of PV plant produc0on schedule aher applying an op0miza0on algorithm No aging cost Aging cost is considered 14

15 Conclusions ü It is necessary to include a reliability analysis when BESS opera0on strategies are determined. ü Cycle and calendar aging must be taken into account. ü When deciding the investment cost for bajery replacement, an op0miza0on analysis is required. ü Appropriate modeling techniques have been described. 15

16 RELIABILITY-BASED OPERATIONAL STRATEGIES FOR ENERGY STORAGE SYSTEMS IN TRASPORT SYSTEMS APPLICATIONS Prof. Evangelos Dialynas 16