2.2.6. Lubrication

Generally speaking, there are three different lubrication principles for high speed rolling element bearings in FESS:


For the considered use-cases are only oil and grease lubrication are relevant, as solid lubrication is mainly used when ambient pressures below 0.1 to 0.01 μbar are required [34]. Furthermore, oil and grease have superior service life for application in FESS compared to solid lubrication, and both are applicable for the considered use-cases. It must be noted that special vacuum grease/oil must be used in order to avoid outgassing, which has a detrimental effect on vacuum quality and may even lead to system failure.

Grease lubrication represents the most commonly used lubricating concept because it requires the least constructive and financial effort to implement, but shows some drawbacks regarding service life compared to oil lubrication [35]. Usually, fresh grease is stored in cartridges and extracted on demand. Standard recommended shelf life for grease in closed and sealed cartridges goes up to five years when stored properly [36]. During operation, service life depends strongly on applied loads and temperature. Above the permissible continuous maximum temperature for a specific grease, a temperature rise of 15 ◦C cuts grease service life in half [32]. Therefore, thermal managemen<sup>t</sup> is crucial for FESS. In order to reach high service life, ongoing maintenance and grease change is required.

Oil lubrication is superior to grease with respect to service life. The used oil can be filtered continuously, thermally conditioned and may easily be changed on demand. Initial costs and effort to implement an oil lubrication circuit are significantly higher compared to grease. Due to the operation under vacuum, the lubrication concept must meet special requirements. Though the functionality of oil lubrication in FESS has been demonstrated in various research projects [37], these systems are not available off-the-shelf.

#### 2.2.7. FESS Service Life

Based on the reference system presented in Table 6, Figure 9 summarizes the main influencing factors and their effect on bearing and lubrication service life.

In this example, grease lubrication and operation at the maximum continuous temperature limit are assumed. Therefore, every increase of 15 ◦C decreases lubrication service life by 50% until the maximum permissible operating temperature is reached (not shown in Figure 9). In short, FESS service life > 25 years is feasible with only minor maintenance effort.

**Figure 9.** Major influencing factors (temperature, imbalance force, operational speed, weight compensation) on bearing and lubrication service life for the reference case listed in Table 6.

#### **3. Use-Case Analysis and Results**

In order to dimension any energy storage device accordingly, the predictability of the duty cycle is absolutely essential. Hence, one of the best predictable use-cases—public transport bus service—was chosen. To be more precise, the case of fully electrified buses with on-board energy storage (no hybridor trolley-variants) is considered. To specify the requirements for the energy storage onboard the bus, or possibly inside the charging station as buffer storage for grid load mitigation, the city of Graz in Austria is used as an example:

According to the public transport company (https://www.holding-graz.at/graz-linien) of Graz in Austria, 151 buses (currently mainly equipped with diesel engines) drive an average of 25,000 km per day. In total each bus travels around 0.5–1 million kilometers during its expected lifetime of 10–15 years. Using a representative urban city bus route ("Route 63" in Graz), the typical energetic requirements for a 12-m-bus were derived based on experiences gathered by the operator, and are shown in Table 7.


**Table 7.** Energetic properties of the reference use case for energy storage analyses.

\* ... Value based on operator experience, including energy demand for heating and cooling.

Operating time during a whole day is around 16–18 h, so around 300 kWh storage would be needed to drive a whole day without any charging stops. For the most part of the day, there are six buses on the route simultaneously. Therefore, a bus arrives at the 'end-stop' every 10 min and (depending on traffic) a few minutes are left before it has to leave again. This time (0–5 min) could be used as a 'charging-window'. Assuming a 2-min charging window at the 10 min end stop (The actual duration of the end stop may vary depending on traffic; hence, a minimum of 2 min is assumed for charging.), an average power of 540 kW would be needed to transfer the previously calculated 18 kWh. Figure 10 shows two electric buses during operation in Graz.

**Figure 10.** Fully electric buses operating in Graz, Austria: (**a**) Bus equipped with supercaps at the pantograph charging station (Route 50); (**b**) Bus with Lithium-Ion batteries (Route 34).

## **On-board Energy Storage:**

In principle, three different scenarios for the in-bus (onboard) energy storage have to be considered:


Due to the limitations of the 'small/minimum storage' scenario and the exclusion of hybrid solutions, for further calculations, only case 1 and 2 are taken into account.

#### **Charging Station Energy Storage:**

In terms of the integration of renewables into the grid, two aspects are important:


This can be achieved by utilizing an energy storage device located directly at the charging station, as it was proposed and tested in References [11,16,39] and depicted in Figure 11.

**Figure 11.** Electric vehicle charging station concept with built in buffer storage. (Photos with kind permission by SECAR Technologie GmbH and PUNCH Flybrid).

Given the previous example of 2-min-long 540 kW charge pulses every 10 min, an energy storage enhanced charging station with 90% efficiency would smooth out the power drawn from the grid to a continuous value of around 130 kW. This would lower the demand on the grid by avoiding possible (thermal) overloads of the powerlines and keeping voltage and frequency within the required specifications. During an entire day, around 100 charging processes are performed accumulating to about 35,000 cycles per year. An additional advantage of storage enhanced charging station is the reduced grid power compared to overnight charge: A single bus needs about 300 kWh per day. Therefore, charging all six buses in about 5 h overnight needs about 360 kW—around three times more compared to the previously calculated 130 kW.

The high demands regarding power, cycle life and energy content of energy storage for professional use in public transportation require careful evaluation of the underlying aging processes, as discussed in Section 2. For the considered on-board use-cases ('medium' and 'large' energy storage) supercaps and flywheels were ruled out for the following reasons:


#### *3.1. On-Board 'Large' Energy Storage*

For the given the example, a 300 kWh energy storage device is needed to allow operation for an entire day. To keep the weight within limits energy storage with high specific energy should be used, e.g., an NCA or NMC Li-Ion battery. Since the battery deteriorates during usage, and 300 kWh is needed at EOL, the BOL capacity must be accordingly higher. Using a capacity deterioration of 33% to define the EOL state, the BOL capacity must be increased to 450 kWh.

With respect to lifetime analysis, cycle life will be dealt with at first. If the battery is charged overnight, it is cycled only once a day, resulting in a high DOD each day. Over the duration of 10 to 15 years, this leads to about 3000–5000 cycles, or an energy throughput of 1–1.6 GWh. Normally, high energy cells are not capable of sustaining that many cycles. Typically, they are able to deliver only 500–1000 cycles (100% DOD), which for the 450 kWh battery would give an energy throughput of about 0.23–0.45 GWh until EOL. Therefore, they would have to be exchanged one or more times over the lifetime of the bus. However, if the operating regime is changed to intermediate charging at the end station, the DOD is significantly lowered. In the given use-case the energy transferred for a single cycle is 18 kWh. Compared to the battery capacity of 450 kWh this corresponds to a DOD of only 4%. As previously shown, the possible energy throughput during a battery's lifetime depends tremendously on the DOD—for some batteries, the manufacturers claim an inverse proportional behavior on DOD [10,14]. That means, for a DOD of 4% an increase of cycle life by a factor of 25 would be achievable, resulting in a possible energy throughput of 6–11 GWh, far exceeding the needed 1–1.6 GWh previously shown. So, even if the actual battery does not scale as well, a (cycle) lifetime of 10 to 15 years should still be feasible.

Of course, another possibility is to use cells with higher cycle life to enable the possibility of overnight charge. Those could either be a di fferent battery technology (e.g., LTO or LFP), or they could still be of NCA or NMC type (but optimized for longer life). In both cases, the disadvantage would be a higher battery weight, due to lower specific energy of the cells.

Taking into account the calendar life as well, a significant additional decrease in capacity would occur during those 10–15 years. Hence, most likely, the battery would still need to be replaced after about 5–10 years. Still, to reach such high lifetime in this heavy-duty application thermal conditioning to low operating temperatures (around 20 ◦C) is mandatory. Table 8 summarizes the above elaborations.


**Table 8.** Comparison of charging regimes for the use-case of 'large' on-board energy storage.

a how long the storage is expected to last in terms of calendar years. Excludes influence of charge/discharge. b how long the storage is expected to last in the real application, including influences of charge/discharge and device temperature based on an EOL capacity fade of 33%.

#### *3.2. On-Board 'Medium' Energy Storage*

Using energy storage with less capacity can save cost and weight. For the example considered, a BOL capacity of 90 kWh (80% reduction in respect to the previous example) is assumed. Given the recharge power of 540 kW, this corresponds in a charging C-rate of 6, too high for a 'high energy' optimized battery. Moreover, since the energy storage has less capacity than in the above example, the cell must have a much higher cycle life, as is the case with LFP and LTO cells. These cells have less specific energy, reducing the possible weight savings, but still, an improvement by a factor 2–3 would be possible compared to the 'large' energy storage case.

Using an AMP20m1HD-A cell as an example (LFP cell from A123 [9]), the cell loses 10% capacity after 2700 cycles (100% DOD, 23 ◦C, 5 C charge- and 1 C discharge-rate). Since the DOD of the considered use-case is only 20% (18 kWh/90 kWh), it can be assumed that after an energy throughput of 1–1.6 GWh (cycling during 10–15 years) the cell has lost about 10–15% capacity. Due to the calendar aging additionally about 15–25% capacity is lost after 10–15 years (at 23 ◦C), resulting in about 25–40% overall capacity loss. Still, even the 40% loss after 15 years (with ~54 kWh remaining) would enable the bus to drive 3 whole rounds. Table 9 summarized the key findings explained in the paragraph above. 


**Table 9.** Summary for the use-case 'medium' on-board energy storage.

a how long the storage is expected to last in terms of calendar years. Excludes influence of charge/discharge. b how long the storage is expected to last in the real application, including influences of charge/discharge and devicetemperaturebasedonanEOLcapacityfadeof 33%.

#### *3.3. End-Stop Charge Station Energy Storage*

In contrast to the on-board energy storage, which is charged once a day or every hour at the bus-stop, the charge station's energy storage is cycled every 10 min, resulting in about 35,000 cycles per year. Assuming a charge transfer e fficiency of 90%, during the charge duration of 8 min 127 kW are drawn from the power grid, charging about 15 kWh into the energy storage. Afterwards, the energy storage is discharged within 2 min with a power of 413 kW. With the additionally 127 kW still drawn from the grid, the bus is charged with the desired 540 kW, as shown in Figure 12.

Using a chemical battery as bu ffer stationary storage in this scenario, a technology with very high cyclability (e.g., LTO) is necessary. For example, a 100 kWh battery built with Altairnano's LTO technology [10,40] would have 25 years calendar life or sustain about half a million cycles at 15% DOD (both values at 25 ◦C). In combination, this would give about 10–15 years of service life. Using supercaps could increase this interval to about 15–25 years [8]. However, like for the on-board energy storage scenarios, the thermal conditioning to 20–25 ◦C (max.) is absolutely necessary to achieve those long lifetimes for electrochemical energy storage.

Alternatively, using a FESS could tremendously extend the achievable service life, well beyond 25 years. Of course, there are some components, which may need to be replaced during the systems life span, like grease and oil in the case of mechanical bearings, the necessary maintenance of the vacuum pump or the exchange of electronic components. However, the main parts (flywheel mass, housing, electric motor), which also represent the main cost factors of the energy storage system are characterized by excellent longevity and do not need to be replaced. An additional advantage of FESS compared to electrochemical energy storage systems is the insensitivity to temperature—especially regarding lifetime—enabling the usage of simple water cooling circuits instead of costly chilling systems.

**Figure 12.** Grid load and charging power level of the load cycle for the "end-stop charge station use-case" shown for two consecutive charging cycles.
