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this cycling operation of one year due to the losses especially during long term high speed idling.
Fig. 2-7 Calculated energy saving Esave and grid feed-in Efeed-in for one year operation of the residential 5 kWp PV system with annual generation of 5030 kWh and annual
elec-tricity consumption demand of 4135 kWh (reference: household without PV system or storage devices)
Fig. 2-8 Calculated self-sufficiency ξ, self-consumption and the energy efficiency of the flywheel for one year operation of the residential 5 kWp PV system with annual generation of 5030 kWh and annual electricity consumption demand of 4135 kWh
(ref-erence: household without PV system or storage devices)
FESS Parametric Study
2. Power Flow Analysis of Residential PV Systems with Flywheels
25
Fig. 2-9 depicts the self-sufficiency ξ of the PV system. For a fixed Pe, with the increasing C, ξ increases at first and then saturates to a certain value due to the poor PV generation in the winter. Even for the hypothetic case of the FESS without any auxiliary power de-mand (Pe = 0) and a very large capacity C, the self-sufficiency is limited to approx. 65 %.
With increasing Pe, ξ drops, as Pe has to be satisfied by discharge of the FESS when op-erating in consumption dominant mode, thereby the usable amount of energy is re-duced. Actually, Pe increases with higher capacities depending on the technology ap-plied. Therefore, an oversized capacity of the FESS is not reasonable.
The self-consumption increases with higher capacity and Pe, as shown in Fig. 2-10. This graph also provides an idea how the FESS consumes the energy from PV generation. For example, with C = 8 kWh and Pe = 400 W, 50 % self-sufficiency can be obtained from Fig. 2-9. But nearly 90 % of the PV generation is consumed from Fig. 2-10. The high self-consumption cannot be treated as a benefit here, as the generated energy is con-sumed by the losses of the flywheel.
Concerning the energy efficiency of the FESS, Fig. 2-11 illustrates that η is never higher than 60 % even for the case of Pe = 0, which considers solely air friction losses and en-ergy conversion losses in the E-machine. Only for low capacities increases with C.
With higher capacities, the windage losses also increases, declines gradually due to the long term high speed idling. It should be mentioned that the efficiency can be im-proved by changing the technology applied, especially by reducing the pressure level in the containment or reducing the auxiliary power Pe (e.g. by using superconducting magnetic bearings). However, both technologies will increase the costs.
Fig. 2-9 Calculated self-sufficiency ξ of 5 kWp PV system with a FESS for varying capaci-ty and auxiliary power demand
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Fig. 2-10 Calculated self-consumption γ of 5 kWp PV system with a FESS for varying capacity and auxiliary power demand
Fig. 2-11 Calculated energy efficiency η of the FESS used in 5 kWp PV system for vary-ing FESS capacity and auxiliary power demand
2.3 Discussion of Flywheel vs. Batteries
Based on the simulated case of a 5 kWp PV system with 3 kWh FESS in a one-family house, we can summarize:
The self-sufficiency (indicating also a percentage of energy saving) of the studied PV system can be improved from 36.8 % to 51.9 % by a FESS, meaning also an extra ener-gy saving of 15.1 % compared to a household without PV system or storage devices. But this extra benefit is achieved by sacrificing the grid feed-in energy, which is 2.5 times the amount of the extra saving. Therefore, the benefit of using a flywheel can be only achieved if the feed-in price is 2.5 times lower than the electricity price, regardless of the capital costs of the flywheel.
Furthermore, the flywheel is 12 % of the time fully charged and 55 % at standstill. The utilization for energy conversion and transfer is rather low. The overall energy efficiency
2. Power Flow Analysis of Residential PV Systems with Flywheels
27
is 40 % for the investigated one-year operating cycle. The main reason is the high self-discharge due to internal losses (approx. 232 W, which is 7.7 % of the maximum stored energy per hour) and the long term idling operating cycle.
As a comparison, the efficiency of commercial Li-ion battery systems (including inverter and charge regulator) can reach 70 … 85 % [30] with the self-discharge in the range of 0.03 … 0.33 % per day [7]. Concerning the costs, the retail price for a complete battery storage system including an inverter is approx. 2500 €/kWh [36]. The flywheels are not commercialized, a cost range of 200 … 150000 €/kWh is given in [7]. Even though the Li-ion batteries have a lower cycle life (5000 cycles [36]) than flywheels (105 … 107 cycles [9]), the diurnal cycle requirement in the residential PV system can still assure a 10 … 15-year service life of the batteries. Therefore, high cycle endurance of flywheels cannot achieve significant benefits here.
Based on the analysis above, the author concludes that conventional flywheels are nei-ther technically nor economically competitive with commercial batteries as long term storage devices in household PV systems. The high self-discharge is the critical disad-vantage of flywheels. This shortage must be overcome if using flywheels for long term storage. One possibility is to develop low loss flywheels, which relies on low loss levita-tion technology (e.g. superconducting magnetic bearings) and high level vacuum tech-nology (e.g. turbo-molecular pumps). Another solution regarding the high self-discharge is to reduce the idling time of the operating cycle, using a highly frequent charge/discharge cycle. In this way, the energy losses in a short time can be neglected compared to the energy converted during the frequent charge/discharge. That is also the reason why flywheels are suitable for short term storage with high cyclic and high power demands.
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3. Power Flow Analysis of a Tramway Sys-tem with an Onboard Flywheel
Flywheels can be used in the city railway systems (Tram, subway, E-bus, etc), where a highly fluctuating power demand is usually observed due to the frequent starting and braking periods of the vehicles. These fluctuated power peaks cause two main problems.
Firstly, they evoke high current in the transmission lines, leading to high losses. Second-ly, the regenerative energy during braking, if they are not able to be fed back into the grid nor shared with the vehicles in vicinity, have to be converted into heat by the resis-tor and wasted. By using sresis-torage devices, the power demand can be smoothened, as they can accumulate the recovered energy and save it for the use in the next traction period, avoiding the energy wasting in the braking resistor and reducing the power de-mand from the transmission lines for the traction. As a result, the total energy consump-tion of the vehicles can be reduced.
Storages can be installed onboard or off-board in substations. If installed in substations, one single storage device can supply all the pass-by trains. However, the losses in the transmission lines are inevitable. The onboard storages can minimize the transmission losses and more flexible. It also offers the possibility of centenary-free operation to re-duce the visual impact. The discussion in this chapter will focus on the onboard flywheel instead of the stationary ones in the substations.
Various storage devices are developed for onboard storage purpose: batteries, electric double layer capacitors (EDLC), as well as flywheels. Batteries and EDLCs are more widely used than flywheels so far. Batteries have high specific energy (at least 10 times higher than EDLCs [37]), but present two disadvantages: lower specific power (0.5 kW/kg [37]) and shorter cycle life (approx. 2000 cycles [38]). Therefore, batteries are more suitable for centenary-free operation as an energy container rather than an energy accumulator for the braking energy recovery. EDLCs are suitable for energy re-covery due to the high specific power (approx. 5 … 10 kW/kg [37]) and long cycle life (106 cycles [38]). Therefore, the trend now is to combine both technologies.
Flywheels have comparable performance as EDLCs. But EDLCs have degradation prob-lems. According to [37], accepted levels of performance degradation for EDLCs are 100 % resistance growth and 20 … 30 % capacity loss over 10 years and half million cycles,
3. Power Flow Analysis of a Tramway System with an Onboard Flywheel
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whereas, flywheels usually have a lifetime of 20 years without degradation problems.
However, flywheel systems are more complicated and have higher safety risk. Both technologies have pros and cons. As an important aspect for the comparison between these two technologies, an energetic performance evaluation based on the measured driving cycle of a tram is presented in this chapter, which is summarized from the pub-lished paper [75]. The evaluation modeling is similar to the system in Chapter 2.
3.1 System Description and Modeling System Operating Strategy 3.1.1
According to the accelerating and decelerating demand, two operating modes with power flows between each power source and load are defined, as shown in Fig. 3-1. For the consumption mode, the power supplied by FESS has the highest order of priority. If the FESS is fully discharged or the power demand exceeds the maximum power of FESS, the power is provided by the grid. When the tram decelerates, the regenerated energy feeds the FESS in the first priority. When the FESS is fully charged or the generated power exceeds the power limit of the FESS, the excessive energy is fed back to the grid.
Here, a feed-in power limit of the grid is defined in order to incorporate the feedback energy of other sources. Therefore, the power above this limit should be consumed by the braking resistor.
Tram
FESS Resistor
Grid
③
①
②
Tram
FESS Resistor
Grid
①
②
a) b)
Fig. 3-1 Tram system operating modes: a) consumption mode, b) generation mode.
(Numbers show the priority of the power flow from 1 to 3 in decreasing order. )
By defining the output power of the FESS as PFW (PFW > 0 for discharge; PFW < 0 for charge) and the power consumption from the grid as Pg (Pg > 0 for consumption; Pg < 0 for grid feed-in), the system energy balance can be represented by
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Tram,B res FW g
P P P P , (3-1)
where Pres is the power consumed by the braking resistor, which is only active (Pres > 0) when the power limit of Pg is exceeded in generation mode: Pg < Pg,max < 0. For other cases, Pres = 0. The PTram,B is the biased power of the tram, which is the sum of the pow-er of the tram PTram and the additional power demand Pe in FESS (explained in Chapter