Assessment of the use of vanadium redox flow batteries for energy storage and fast charging of electric vehicles in gas stations

Highlights

• Assessment of Vanadium Redox Flow Battery use for EV fast charge in gas stations.

• This novel system proposal allows power peak shaving and use of deactivated gas tanks.

• Philosophy allows seamless business transition towards the Electric Mobility paradigm.

• Project is technologically and economically viable, although with long payback times.

• Future Cost cuts due to technology maturation will consolidate project attractiveness.

Abstract

A network of conveniently located fast charging stations is one of the possibilities to facilitate the adoption of Electric Vehicles (EVs). This paper assesses the use of fast charging stations for EVs in conjunction with VRFBs (Vanadium Redox Flow Batteries). These batteries are charged during low electricity demand periods and then supply electricity for the fast charging of EVs during day, thus implementing a power peak shaving process. Flow batteries have unique characteristics which make them especially attractive when compared with conventional batteries, such as their ability to decouple rated power from rated capacity, as well as their greater design flexibility and nearly unlimited life. Moreover, their liquid nature allows their installation inside deactivated underground gas tanks located at gas stations, enabling a smooth transition of gas stations' business model towards the emerging electric mobility paradigm. A project of a VRFB system to fast charge EVs taking advantage of existing gas stations infrastructures is presented. An energy and cost analysis of this concept is performed, which shows that, for the conditions tested, the project is technologically and economically viable, although being highly sensitive to the investment costs and to the electricity market conditions.

Introduction

The disruptive increase of urban traffic along the last decades is posing serious sustainability concerns, mainly those related to urban air quality and GHG (greenhouse gases) emissions, as well as the excessive dependency of developed economies on fossil fuels. It is expected that in 2030 the transportation sector will be responsible for 55% of total oil consumption [1]. It is also expected that the population will grow 1.7 times and the number of cars even more (3.6 times) between 2000 and 2050 [2]. In this context, the current policies promoting emissions reduction and the improvement of the energy efficiency of ICE (Internal Combustion Engines) are contributing to palliate these issues [3]. Various strategies have been explored along time to address these issues, such as engine downsizing achieved with turbo-charging [4], the strategy of over expansion explored by the authors [5], [6] and used in several efficient hybrid powertrains or waste energy harvesting such as exhaust thermal energy recovery in form of Organic Rankine Cycle or Seebeck effect thermoelectric generators [7], [8].

Nevertheless, the increase of the overall efficiency of conventional powertrains does not seem sufficient by itself to achieve the efficiency and emissions goals set by national and international agreements, nor does it improve the desired diversity of energy sources. Nowadays, the main alternatives to the traditional ICE are the PHEVs (Plug-In Hybrid Electric Vehicles) and the full EV (Electric Vehicles) [9]. These alternatives allow the reduction of the global fossil fuels consumption that is allocated to the traditional transports systems and are a key technology to the future smart grids [10]. Some of these alternatives are now available in the market with substantial success [11], such as Toyota Prius (PHEV) or the Nissan Leaf (EV). These vehicles are globally more efficient than ICE vehicles, mainly under urban traffic since they have no idling losses, they have good low end torque without the need for inefficient clutching, and they can recover some of the kinetic energy lost during the braking [3], [11]. In Ref. [12] a comparative environmental life cycle comparison between conventional and electric vehicles has been presented. As an example, using EVs, the global GHG emissions can decrease from 10% to 24% when compared with conventional diesel or gasoline vehicles. In Ref. [13] a study highlighted the EV as a means to contribute to the overall reduction of the fossil sources and energy used for transportation, although certainly this will depend on the electricity production performance.

Unfortunately, the success of PHEVs and EVs is currently hampered by some notable disadvantages, mostly related with energy storage and power grid charging [14]. The main disadvantages are their typically low autonomy (usually up to 150 km) which results from the low energy density of current battery technologies and the long time required to perform standard battery charging processes (typically, a full charge will require around 8 h to complete) [11], [15]. The combination of these two factors is known to induce the so-called range anxiety phenomenon which, along with the high cost of batteries is preventing the wide adoption of electric mobility [16]. A range extender unit may be added to the powertrain to prevent this, and in fact the authors have confirmed the merits on a Life Cycle basis, of efficiency-oriented range extenders [17], but the use of such systems increases design complexity and cost, as the price tag of some existing models incorporating range extenders indicate.

In order to minimize some of the aforementioned shortcomings related to energy storage, some EVs allow to perform a fast battery charging. The CHAdeMo (CHArge de MOve) protocol [18] is one of the most popular DC fast charging protocols in electric mobility, normally displaying a maximum power output of 50 kW. Fig. 1 shows an example of a CHAdeMO fast charging station developed by a partner (PETROTEC) of the team. With this charging mode the battery of many existing models can be charged up to 80% of their State-of-Charge (SoC) in less than half an hour [19]. This substantially reduces the inconveniences associated with small range, provided that fast charging stations are available along the main roads. Of course, BEVs are not practical for frequent long trips due to the need for frequent charging stops. Nonetheless, it would be highly valuable for electric mobility that these long trips would be possible to do if necessary. The range of mass market Battery Electric Vehicles is often around 100–150 km, so the suitable distance between two consecutive charging stations should be lower than that distance to allow for occasional long trips.

Unfortunately, the high power output required by these chargers is demanding in terms of local infrastructure. A high power consumption plan must be contracted with the electric grid service provider, representing a substantial fixed cost even without any energy consumption. Moreover, EV charging demand will normally occur at daytime, coinciding with costly electrical peak demand periods.

Fortunately, many of the aforementioned disadvantages of fast charging may be averted by decoupling grid consumption and the consumption due to vehicle charging by means of stationary energy storage systems. In fact, the energy needed for high power vehicle charging may be stored previously and more gradually (with lower average power) at off-peak demand schedules than in the case of direct grid vehicle charging. This allows reducing both the installed power consumption limit and the average cost of electricity. Also, power quality problems associated with power grid voltage, stability and frequency are minimized [20]. In this context, the present work explores the use of a specific energy storage technology to perform EV fast charging during daytime using electricity previously stored during low demand periods. Moreover, the proposed energy storage technology could also be integrated into microgrids, to store the energy produced from renewable power sources contributing to smooth their intermittent production and adapt it to power demand [21].

The load levelling process and the peak shaving process rely on the storage of energy during low demand periods, releasing that energy when the electrical load is high [22], [23]. The main goal of the load levelling process is to stabilize the electrical load, avoiding fluctuations in the consumed power, while in the case of peak shaving process the main goal is to use the stored energy solely to remove the load peaks consumption. For both processes, the energy stored during the night is equal to the energy supplied by the storage system during the day. The comparison between peak shaving and load levelling is illustrated in Fig. 2.

Typically, these two processes are implemented in low output power applications, such as domestic grids or small factories with a few kW of power. They have several advantages, the first of all being the reduction of the maximum power consumed from the power grid and consequently the reduction of the installed power, which results in lower prices [24]. Secondly, it permits a better management of the energy demanded from the power grid, taking into account the different energy prices depending of the schedule, because it is possible to buy cheaper power during off-peak periods, such as during night-time [24]. Thirdly, it permits a greater incorporation into the grid of energy derived from renewable sources like solar and wind, which are unpredictable sources, often with the peak power generation occurring in counter-cycle with demand. This means that the availability of an energy storage buffer will avoid wasting the energy produced during low demand periods storing it and releasing it later during high demand events. This will enable a real substitution of electricity obtained from fossil fuel by electricity from renewable energy sources [25], [26].

Nowadays, reversible hydroelectric power plants are being often used since they can use the excess of energy produced by renewables (generally the wind energy produced during night hours) to pump water back to the hydroelectric dam, which creates a gravitational energy storage. However, this resource is not always available or sufficient to solve the problem and so, the integration of large scale batteries systems in the electrical power grid seems to be a good solution for complement this energy storage system.

There are several energy storage technologies that can be used for load levelling and peak shaving processes besides the pumped hydro storage. They are compressed air storage and batteries. Regarding for batteries, many research groups have been studied the use of lead acid [27], [28], sodium sulphur (NaS) [22], lithium ion [29] and also RFB (Redox Flow Batteries) [30] for these applications.

Many of the aforementioned systems have requirements not easily achieved for the application proposed in this work. Among the various battery technologies, the RFB have several advantages over the remainders, namely because their energy capacity is uncoupled from their rated power [31]. This is so because energy depends mainly on the amount of electrolyte stored, while rated power is a function of the cell stack. Other advantages of these batteries are related with their liquid nature and their storage (in tanks), which can be of any shape. In Refs. [32] and [33]the recent developments and studies of RFB concerning electrolytes, electrodes, membranes, and aqueous and non-aqueous systems have been reviewed. There are many types of RFB with various redox couples used, however, the VRFB (Vanadium Redox Flow Battery) is currently among the most studied and promising technologies of this kind. These batteries have the advantage of using the same chemistry in both half cells. The main advantage of this is that the cross-contamination of the electrolytes will not render the resulting mix unsuitable for the function as would be the case of electrolytes with different chemistries [34]. This is one of the main reasons for their fairly extended life even when compared with the latest Lithium ion battery chemistries. As main disadvantage, complete VRFB systems are still expensive, although the growing maturity of this technology and its attractiveness as an enabler for the wide adoption of intermittent renewable sources is likely to decrease its cost in the midterm [35], [36], [37].

The recent economic crisis affecting several western economies was accompanied by a reduction in the demand of transportation fuel [38], this reduction is showed in Fig. 3 and it can be seen that in the European Union the fuel consumption dropped by almost 4% between 2007 and 2011. In the same period, some of the most pronounced drops occurred in Ireland and Spain, which reduced around 23% and 15%, respectively. Other sharp reductions can be also observed, like the one occurred in Greece, which reduced about 17% between 2009 and 2011, and in case of Portugal there was a sharp decline of about 7% between 2010 and 2011.

As an alternative to the costly and laborious deactivation/disposal of surplus large fuel storage tanks in gas stations, a retrofit of these deposits could be performed, adapting them for VRFB electrolyte storage and using the storage system for EV fast charging with the strategy explained before. One merit of such an approach would be to easily obtain EV fast charging spots in places which are already strategically located for vehicle traffic, optimizing otherwise wasted space and infrastructures and complementing the ICE vehicle fuel supply business with the emerging plug-in vehicle charging business in one place.

Following previous work by the authors in electric mobility and gas station equipment, including a review article on the VRFB technology and its prospects to the electric mobility area [33], the present work assesses this philosophy by carefully describing the operating principles of a VRFB, the use of energy storage technologies for load levelling and peak shaving and by performing a draft design of a VRFB system capable of fast charging simultaneously two electric vehicles. This design is then analysed in terms of energy performance and economic viability.

The operating principle of RFBs is partly similar to the operation of a conventional battery, but a major distinction is the fact that the energy storage unit (the active materials) is physically separated from the energy production unit (the cell stack) [39]. So, in a VRFB the active materials are not permanently sealed inside the cell (like in a conventional battery), but are stored separately in tanks and pumped into the cell according to the energy demand [34].

This process is represented in Fig. 4, which represents the two tanks, one for positive electrolyte (cathode) and other for negative electrolyte (anode), the cell and the pumps [39]. When the liquid electrolytes are injected into the cell an electrochemical reaction (oxidation–reduction or redox) occurs, with movement of electrons along the electric circuit, as there is an exchange of ions through the membrane to maintain charge neutrality between the different ionic solutions [31].

The positive and negative electrodes in vanadium redox flow batteries are typically carbon based materials, such as carbon or graphite felts, carbon cloth, carbon black, graphite powder and so on [40]. These electrodes have shown a good potential in terms of operation range, a good stability and a high reversibility. Similarly to other battery technologies, the electrodes are a very important component on the performance of the vanadium redox flow batteries.

There are many types of redox flow batteries, such as: the ZBB (zinc–bromine) [41]; the PSB (polysulfide-bromide) [42]; the ZCB (Cerium–Zinc) [43]; and the (Vanadium Redox Flow Batteries) VRFB, which include the first generation (G1 – the all vanadium system, normally called VFRB (Vanadium Redox Battery) in the literature) and the second generation (G2 – the vanadium bromide system) [33], [44].

The G1 is now the most studied technology and involves solely vanadium species in both half cells at different valence states. It operates in an electrochemical couple based on two different reactions of vanadium ions in a dilute acid solution. This is possible because vanadium oxide is a stable material in four different valence states [45].

The cathodic and anodic reactions can be represented as follows [46]:

VO₂⁺ + 2H⁺ + e⁻ ↔ VO²⁺ + H₂O E⁺ = 1.00 V

V²⁺ ↔ V³⁺ + e⁻ E⁻ = −0.26 V

And the overall reaction is [46]:

VO₂⁺ + 2H⁺ + V²⁺ ↔ VO²⁺ + H₂O + V³⁺

The G1 has an advantage relatively to the other redox flow batteries, which is that in the event of a cross mixing between the two liquid electrolytes, the regeneration of the solution may be performed simply by recharging the fluids, unlike systems with different metals in which the mixed liquids would have to be replaced or removed and treated externally or disposed [34].

Unfortunately, VRFBs still have a low energy density when compared with conventional batteries. This is due to the maximum concentration of Vanadium that can currently be dissolved in the supporting electrolyte. In the case of the G1 technology, typically the maximum vanadium ion concentration is 2 M or less, which corresponds to an energy density of 25 Wh/kg or 33 Wh/L, and that concentration is limited by the stability of the V5+ ions at temperatures above 40 °C and the solubility limit of V2+ and V3+ ions in supporting electrolyte at temperatures below 5 °C. Nonetheless, several studies have been made in the last few years testing the incorporation of additives in the positive electrolyte in order to inhibit the vanadium precipitation and enable the use of higher concentrations and thus increase the energy density.

The G2 technology employs a vanadium bromide solution in both half-cells and shares all the benefits of the G1 technology, including the fact that the cross contamination is eliminated [47]. Since the bromide/polyhalide couple has lower positive potential than the V(IV)/V(V) couple, the bromide ions will preferentially oxidize at the positive electrode during the charging [48].

The cathodic reactions of this technology are as follows [49]:

ClBr₂⁻ + 2e⁻ ↔ 2Br⁻ + Cl⁻

or

BrCl₂⁻ + 2e⁻ ↔ Br⁻ + 2Cl⁻

And the anodic reaction is as follows [49]:

V²⁺ ↔ V³⁺ + e⁻

However, the G2 technology has a disadvantage, which is the propensity for the formation of bromine vapours during charging. This requires the use of expensive bromine complexing agents which limit the attractiveness of the G2 technology [48].

Since the G2 technology is not yet in a mature state and the G1 technology is the most studied and there are already several companies commercializing it [50], [51], [52], [53], [54], [55], the present work has focused on this technology. In Fig. 5 a typical charge/discharge cycle of a VRFB at 40 mA/cm² is shown [56]. It can be seen that the maximum voltage during charge is 1.74 V, while during discharge the voltage varies from 1.42 V down to 0.8 V. However, it can also be seen that below approximately 1.15 V, the slope of the curve becomes very sharp, which means that the SoC is close to zero.

Currently there are already several VRFB battery manufacturers and some operating plants worldwide. A previous revision article by the authors provides some insight into this subject [33].

This paper proposes an energy storage technology to be used by electric vehicle fast charging stations to make a peak shaving process, enabling the simultaneous charging of several EVs without having to incur into excessive power availability (e.g. supply capacity) charges.

Following several decades of vehicle traffic, existing gas stations are now generally quite conveniently located and evenly distributed for vehicle access. Therefore, the location of EV charging spots at those places is highly attractive since it will not be necessary to captivate additional real estate to build a network of new charging spots suitably located. An EV owner could charge his vehicle using, for instance, a fast charging CHAdeMo station located at such places in roughly half an hour, enabling the use of EVs for occasional long travels, should these stations be conveniently distributed along the main driveways. Furthermore, gas stations could adapt their business size as a function of the increase of EV market penetration.

Fig. 6 shows a typical steel fuel tank used in gas stations [57], among the various fuel tanks available in the market. Capacities vary from 1000 L of capacity, to the giant tanks with 100 000 L of capacity [57], [58], but one of the most commonly used in gas stations is the 20 000 L tank with around 2.5 m of diameter and around 4.7 m in length. Typically, the gas station underground tanks that are deactivated are not removed and are left unused. It would therefore be advantageous to retrofit these leftover tanks to store the liquid electrolytes. It would inclusively be a way of gradually migrate from a fossil fuel-based business to an electric mobility services business as the market slowly adapts from one paradigm to the other. The present work has considered these tanks for the storage of the liquid electrolytes, but surface tanks are also possible to use and they will facilitate the installation and service of the system.

The shape and material of the tanks raises some difficulties. Since they are typically made from steel [57], [58], the liquid electrolytes cannot be in direct contact with them due to their acid nature. To avoid that contact, the tanks may be coated with an acid resistant material. In such case, each fuel tank can only be used for one electrolyte (positive or negative). Alternatively, one or several smaller flexible tanks made from an acid resistant may be installed inside the steel tanks. In this latter case, two smaller flexible tanks, each one containing a different liquid electrolyte, may be accommodated inside one steel tank.

As can be seen in Fig. 6, the diameter of the main tank opening (only 60 cm) may hamper the insertion of the flexible tanks inside it. Several small solid tanks made from PVC (Polyvinyl Chloride) or other acid resistant material could also be used. However, the best solution seems to be the use of flexible rubber tanks as those firstly proposed by Sumitomo Electric Industries Ltd. These were made specifically to take advantage of the fluidic form of these batteries and to allow their insertion in unused spaces such as underground cisterns, through manholes [59].

Rubber tanks with a shape which reasonably conforms to the interior of the steel tanks should be made, as illustrated in Fig. 7(a), including a support structure to separate both tanks and leaving free space below the manhole to allow the entry of installation and service staff. This structure can be done simply with Landsquare PVC beams and mounted in loco with stainless steel screws.

Another alternative, is to use four tanks (two for positive and two for negative electrolyte). This facilitates their introduction into the fuel tank. On the other hand, this configuration allows the existence of two VRFBs using the same gas tank so it is possible to have one battery to charge EVs and the other one being charged at low power input from the electric grid. The four tank configuration is represented in Fig. 7(b).

To make these tanks, the appropriate rubber should be selected. It must be highly resistant to the corrosion with sulphuric acid under the prescribed concentration. Typically, the two liquids electrolytes (anolyte and catholyte) of a G1 are prepared by dissolving around 2 M VOSO₄ (vanadyl sulphate) into around 5 M H₂SO₄ aqueous solution, to form tetravalent vanadium ions [34], which means that the volume concentration of sulphuric acid may be between 0.005% and 27%.

Table 1 which has been compiled from several references [60], [61], [62], shows the chemical resistance of the most common rubber types, and it can be seen that there are various types of rubber which are resistant of acid sulphuric for the concentrations needed, like the EPDM (Ethylene Propylene Diene Rubber), Butyl and Teflon.

However, the Vanadium oxides are also corrosive and their presence within the electrolyte must be considered, so a rubber which is resistant to both substances is needed. After analysing Table 1, as a first approximation it seems that Butyl rubber will be a good candidate material for the rubber tanks.