2.2.5. Bearings

In most FESS, two fundamentally different bearing concepts are used: Active magnetic bearings (AMBs) and rolling element bearings (REBs). High costs compared to competing energy storage devices represent one of the major market entry barriers for FESS. For that reason, the upcoming sections focuses on low-cost solutions using REBs. Table 5 gives a brief overview of REBs compared to AMBs.


**Table 5.** Comparison of active magnetic bearing and rolling element bearing concepts.

The REB's service life mainly depends on the applied loads. Generally, bearing loads are caused by rotor weight and machine dynamics/imbalance forces. There are di fferent approaches to minimize bearing loads:


Concepts based on active vibration control will not be considered within this publication as they have not reached readiness for marketing in FESS ye<sup>t</sup> [26]. The other three measures are considered and explained briefly in the following paragraph using an example with the specific properties stated in Table 6:


**Table 6.** Properties of a reference FESS for parameter study regarding bearing life.

\* Reference FESS module specifications based on the research project FlyGrid.

Axial bearing loads can, in fact, be almost entirely compensated by a passive magnetic lifting system, as shown in Figure 7, provided the system is designed with a vertical axis of rotation, which is normally the case. As shown in Figure 8a weight compensation is key to reach reasonable bearing service life. An attracting configuration using a ring magne<sup>t</sup> that would directly pull a ferromagnetic steel element on rotor upward has some disadvantages because of high eddy current losses at high rotor speeds. Using an additional permanent magne<sup>t</sup> on the rotor acting as a counter pole, the eddy current losses can be almost eliminated. For the remaining decision of either pulling the rotor on top or pushing it upwards from magnets mounted at the bottom, the latter configuration is preferable, due to

its inherent stability of this configuration taking into account the direction of gravity. A demonstration video showing this configuration is uploaded in the Supplementary Materials of this publication.

**Figure 7.** Cartoon image of low-cost, low-loss bearing configuration, including passive magnetic rotor weight compensation.

However, even if the rotor weight is nearly entirely compensated, imbalance forces remain. Based on a constant imbalance force of 100 N, bearing life for different compensation levels is shown. Without weight compensation, the bearings last only 21 days, but when 95% of the weight is compensated, bearing life increases up to more than 90 years. It must be noted that at compensation levels above 100% one bearing might be completely relieved which may cause slippage of the balls and lead to rapid system failure [31].

A flexible bearing suspension is used to operate the rotor supercritically. This means that at least the first two eigenfrequencies are surpassed and "self-centering" of the rotor occurs. During supercritical rotor operation bearing primarily loads, depend on the bearing seat's stiffness and the rotor imbalance and not on rotational speed [26]. Figure 8b shows the decrease of bearing life from 90 years to 25 years, when the rotor imbalance force is increased from 100 N to 300 N. Higher imbalance requires higher axial prestress of the bearing configuration, which is taken into account. For this study a magnetic weight compensation of 95% resulting in a remaining weight load of 74 N is assumed. Still, it must be mentioned that rotor imbalance may change over time, due to creep, wear or setting of joints.

**Figure 8.** Influence of weight compensation (**a**) and rotor balancing quality (**b**) on bearing life (based on the reference case, shown in Table 6.).

Bearing friction torque is one of the main causes of losses in FESS and is mainly influenced by the following parameters:



In order to calculate the resulting power loss bearing friction torque must be multiplied with the rotational speed.

The influence of bearing size, cage material and weight compensation factor on torque loss is a very complex matter and outside the scope of this publication. Detailed studies on the minimization of FESS bearing losses are available in References [1,32,33]. Still, as lubrication strongly affects FESS service life, the effects of lubricant viscosity and minimum quantity lubrication are discussed in the subsequent Section 2.2.7.
