Critical Technologies 1.2.2
Chapter 7 presents the magnetic bearing system. The configuration of the magnetic bearings are introduced, including one radial magnetic bearing and one combined type
which produces forces in both radial and axial direction. The force and losses in the magnetic bearings are analyzed.
In Chapter 8, the system set-up is presented. The designed rotor, the E-machine and magnetic bearings are assembled together. An inner housing is designed as a support-ing frame of all the components. The main issues concernsupport-ing the component processsupport-ing and the assemble work are presented. Due to the safety consideration, two outer hous-ings are designed as burst containments in case of the rotor structural failure. In addi-tion, the air friction losses of the rotor are calculated for the low pressure in vacuum.
Chapter 9 presents the thermal calculation of the flywheel system. The loss components of the system are summarized. A lumped parameter thermal network is modeled and used to calculate the temperatures and heat flows. The calculation is carried out for two operating conditions: continuous operation and the operation with a fully-utilized duty cycle. In the end, the design of the stator water cooling system is presented.
Chapter 10 … 11 presents the discussions concerning the critical issues for a high pow-er (150 kW) machine and light-weight rotors made of fibpow-er reinforced matpow-erials, as an outlook for the future work.
As the flywheel system designed in Chapter 4 … 9 is a preliminary demonstrative proto-type, in which the power rating is downsized from 150 kW to 35 kVA due to power limit
1. Introduction
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in the EW lab. In order to fulfill the power requirement for the application in the aimed tram system as discussed in Chapter 3, a theoretical design of a PM machine with a full power of 150 kW is proposed in Chapter 10 as a conception for the follow-up design for the onboard application.
As high specific energy and energy density are usually required for onboard application, for which light-weight rotors made of fiber reinforced materials are preferred. Therefore, in Chapter 11 some critical considerations are discussed concerning the design of such rotors as an outlook for the future work. The velocity limitations regarding the stress due to rotation are given, based on which the criteria for the dimensioning of the com-posite rims are introduced. Critical problems regarding the rim-shaft connection are also discussed.
The author would like to express her deeply thanks to her colleague M.Sc. Nicolas Erd for his contribution in Chapter 2 and Chapter 3. Mr. Erd proposed most of the innova-tive ideas for building-up the model in the power flow analysis, and was responsible for all the programing work. Thanks to his work, the worthful results in these two chapters can be obtained. Also many thanks attribute to the author’s ex-colleague M.Sc. Jeongki An, who proposed the design of the E-machine in Chapter 6, including the dimensioning of the geometries and the loss calculation (iron losses and eddy current losses in mag-nets) in FEM model, as well as the stress calculation in the bandage. Based on his work, the author developed further performance analysis (copper losses calculation and field weakening performance analysis).
For building-up the prototype, the author would like to thank Mr. Andreas Fehringer and Mr. Markus Lohnes in the workshop of the Institute for Electrical Energy Conversion, TU Darmstadt, for their efforts in the manufacturing process. With their creative ideas and experienced work, a feasible construction layout of the prototype was proposed and the prototype was successfully built-up. Sincere thanks should also be given to Dr. Yves Ge-meinder and M.Sc. Nicolas Erd, who were responsible for organizing the project, and Dr.
Gael Messager and M.Sc. Daniel Dietz, who are working on the control of the magnetic bearings to realize the levitation of the rotor, as well as Mr. Klaus Gütlich, who sets up all the electrical systems for the testing of the prototype.
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2. Power Flow Analysis of Residential PV Systems with Flywheels
The energy storage devices in residential PV systems can solve the time based mismatch of the power generation and consumption by accumulating the excessive energy and save it for later use when necessary. In this way, the electricity consumption from the grid can be reduced by increasing the self-consumption of the PV generation. Nowadays, the market of energy storage systems for residential PV installations is solely dominated by batteries with a significant trend changing from lead-acid (LA) batteries to lithium-ion (Li-lithium-ion) batteries. Compared to batteries, flywheels have longer lifetime (cycles) with no degradation problems and little environmental impacts. In 1970s, the idea of using flywheels in residential PV systems was already proposed [27], even though be-fore that it has been established that conventional flywheels are neither technically nor economically competitive with batteries [28]. But, according to [28], if a flywheel is de-signed with an integrated functions of DC-AC power conversion and maximum power point tracking (MPPT), the flywheel will be technically and economically competitive with the battery based system: battery plus inverter and MPPT. Based on this idea, a flywheel prototype was built in [29], which has a storage capacity of 4 kWh and power of 500 W. The rotor, which is magnetically levitated, has a maximum rotational speed of 15000 min-1 and operates in vacuum. Experimental measurements show an overall pow-er efficiency of 68 % of the flywheel based system including powpow-er electronics (with approx. 1.5 % loss of stored energy per hour), slightly higher than the battery based system with the value of 65 % (considering 80 % for battery, 85 % for inverter, 96 % for MPPT). This comparison is not valid for nowadays as the efficiency and performance of batteries are considerably increased, e.g. to 85 … 95 % for Li-ion battery [30]. Secondly, the measured efficiency focuses on power conversion efficiency of the prototype unit, instead of considering the cycling operation in the PV system, where the idling losses in the flywheel become critical rather than the power conversion losses for a cycling period of hours.
Therefore, in this chapter, a performance evaluation of a flywheel in the residential PV system will be presented, summarized based on the published paper [77]. The emphasis stays on the overall energy efficiency of the flywheel taking the operating cycle into
ac-2. Power Flow Analysis of Residential PV Systems with Flywheels
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count, and the energy saving potential of the PV system in a time scale of one year. The aim is to give a quantitative evaluation in order to point out the challenges that restrain the usage of flywheels and the technical improvement hints of the flywheels for the use of long term storage in the future.
2.1 System Description and Modeling
The investigated residential PV system is designed to maximize the self-consumption of the PV generation instead of maximizing the feeding into the grid, as this is more profit-able due to the significantly falling grid feed-in tariffs in recent years in Germany [32].