EuCARD-2 Consortium, 2016.
Grant Agreement 312453 PUBLIC 1 / 15
EuCARD-2
European Coordination fo r Accelera tor Research and Development
S e v e n t h F r a m e w o r k P r o g r a m m e , C a p a c i t i e s S p e c i f i c P r o g r a m m e , R e s e a r c h I n f r a s t r u c t u r e s , C o m b i n a t i o n o f C o l l a b o r a t i v e P r o j e c t a n d C o o r d i n a t i o n a n d S u p p o r t A c t i o n
DELIVERABLE REPORT
E NERGY S TORAGE S YSTEMS FOR
A CCELERATORS
D ELIVERABLE : D3.4
Document identifier: EuCARD2-Del-D3.4-Final Due date of deliverable: End of Month 42 (October 2016) Report release date: 25/11/2016
Work package: WP3: EnEfficient Lead beneficiary: KIT
Document status: Final
Abstract:
Energy storage systems are needed in accelerators for different purposes. Often accelerators are operated in pulsed mode, for example Linacs run in the range of 10…100Hz, or synchrotrons are cycled at a lower frequency. The varying load on the grid is not acceptable and a storage system is used to accommodate a constant load. Other purposes include bridging of grid interruptions and energy management. In this report the different applications and technologies for energy storage in accelerators are reviewed.
Grant Agreement 312453 PUBLIC 2 / 15 EuCARD-2 Consortium, 2016.
For more information on EuCARD-2, its partners and contributors please see http://eucard2.web.cern.ch/.
The European Coordination for Accelerator Research and Development (EuCARD-2) is a project co-funded by the European Commission in its 7th Framework Programme under the Grant Agreement no 312453. EuCARD-2 began in May 2013 and will run for 4 years.
The information contained in this document reflects only the author’s views and the Community is not liable for any use that may be made of the information contained therein.
Delivery Slip
Name Partner Date
Authored by R. Gehring, M. Seidel, J. Eckoldt KIT, PSI,
DESY 10/10/16 Reviewed by R. Gehring, M. Seidel, J. Eckoldt KIT, PSI,
DESY 10/10/16 Approved by
WP
Coordinator
M. Seidel PSI 23/11/16
Approved by Steering Committee
31/10/16
Approved by Project coordinator
M. Vretenar 31/10/16
Grant Agreement 312453 PUBLIC 3 / 15 TABLE OF CONTENTS
1. EXECUTIVE SUMMARY ... 4
2. INTRODUCTION ... 4
3. APPLICATION OF ENERGY STORAGE IN ACCELERATORS AND ASSOCIATED REQUIREMENTS ... 4
4. TECHNOLOGIES FOR ENERGY STORAGE ... 7
4.1. MECHANICAL FLYWHEEL SYSTEMS ... 7
4.2. DIESEL GENERATOR ... 7
4.3. CAPACITORS ... 8
4.4. SUPERCONDUCTING MAGNETIC STORAGE (SMES) ... 9
4.5. LIQHYSMES ... 10
5. RESEARCH AND DEVELOPMENT DIRECTIONS ... 12
6. SUMMARY ON ENERGY STORAGE SYSTEMS FOR ACCELERATORS ... 13
7. REFERENCES ... 14
ANNEX: GLOSSARY ... 15
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1. EXECUTIVE SUMMARY
This report summarizes energy storage technology for particle accelerators. The different applications for energy storage in accelerators are described and the suited storage technology is discussed. An outlook is given for storage systems and applications that might become important for future accelerator facilities.
2. INTRODUCTION
New particle accelerator based research facilities tend to be much more productive, but often in coincidence with higher energy consumption. On the other hand resources become tighter, CO2
production from fossil energy carriers must be reduced and nuclear power is associated with risks of operation and disposal of radioactive waste. For the public acceptance of particle accelerator projects it is thus very important to optimize them for best utilization of electrical energy and to show these efforts to funding bodies and to the public. Within the European accelerator development program Eucard-2 [1] the network EnEfficient [2] aims at improving the energy efficiency of accelerators. The main focus of this network concerns the power flow in accelerator facilities, developments of efficient accelerator systems, such as magnets, RF generation, cryogenic systems. But also the aspect of intelligent energy management in a large facility is important. Energy must be stored and provided over differing times and storage systems are important for the overall optimized utilization of electrical power.
3. APPLICATION OF ENERGY STORAGE IN ACCELERATORS AND ASSOCIATED REQUIREMENTS
Particle accelerator facilities for research purposes are complex installations with a power consumption in the range of a few Megawatts up to hundreds of Megawatts, depending on size and application of the facilities [3]. Often accelerators operate in a pulsed regime since the application requires a pulsed particle beam, or because technical limitations require pulsed operation. For example a high gradient and thus cost effective accelerator structure at room temperature is operated repetitively, but per cycle only for a very short fraction of time. The low duty factor is necessary for reasons of cooling and power consumption, both in the structure and the RF source.
Storage rings have a high energy reach and are cost effective compared to linear accelerators, but they must be cycled as well. While the repetition rate of linacs is typically in the range of a few to hundred Hertz, the cycling frequency of synchrotrons is lower, ranging from a few seconds to a few 10 Hertz at maximum. Within a cycle the variation of the energy consumption can be substantial. For example the large J-PARC storage ring cycles every 2.56 seconds and the power consumption varies by 140 MW during that time. A repeated variation with this magnitude cannot be accepted by a public grid system.
Another problem are synchrotrons that operate at lower power, but in the range of the so called flicker frequencies below 20 Hz. Without energy storage these power variations would lead to variation in the intensity of electrical light. This is very disturbing for the public.
Energy storage systems are needed to equalize the consumption on the grid. This application for cycling accelerator systems is the most important motivation to use energy storage systems in accelerators. In this context storage systems are in use already for decades in large accelerator systems, for example at CERN.
Grant Agreement 312453 PUBLIC 5 / 15 Fig. 1: Statistics of trips on the grid affecting operation of the ESRF light source, courtesy J.- F. Bouteille (ESRF).
There exist also other applications with increasing relevance in the future. The energy efficiency of large research infrastructure becomes increasingly important in times of climate change and limited resources. Already a short glitch on the grid can interrupt the operation of a facility.
While most power supplies and other consumers can be switched online quickly, it often takes hours of tedious tuning and startup procedures to bring an accelerator facility up to the full performance it had before the glitch. This means that full power is drawn quickly after the event, while full production can only be achieved after significant time. As a result energy is wasted and the overall efficiency is reduced. The additional cost of personnel and time to recover operation may be even more significant. Fig. 1 shows the trip statistics of the European Synchrotron Radiation Facility ESRF. Many trips are caused by the impact of lightning on the public grid in that area. With energy storage systems as backup supplies such interruptions can be avoided. Of course for this application the capacity of a storage device must be quite significant, for example covering the entire power load of a facility over a few minutes.
With the increase of sustainable energy production in the public grid the fluctuations become larger due to the unstable production of wind and solar energy. A large energy storage system that can supply an accelerator facility for several hours could be used to store energy at times of high supply and to run the facility from this reservoir at times of low supply. In this way the public grid would be less affected in critical times by the potentially significant consumption of a large research facility.
Grant Agreement 312453 PUBLIC 6 / 15 Fig. 2: Typical parameter ranges for different energy storage methods (relevant for this report) in terms of delivered power and duration of power supply.
Table 1: Some examples for energy storage systems of accelerators.
application Cycle period
[s]
total stored Energy [MJ]
Max.
power [MW]
technology Example of application Klystron modulator 0.1 0.09 16 Capacitor European XFEL
modulator
Klystron modulator 0.1 1 25 SMES SMES study
KIT/DESY Cycling synchrotron 1.2 233 40 Flywheel CERN, PS
Synchrotron Cycling synchrotron 1.2 7.5 11 Capacitor Med Austron
Synchrotron
Synchrotron 0.08 0.2 2 White
circuit
DESY Facility backup hours-
days
70GWh 300 LH2 KIT study
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4. TECHNOLOGIES FOR ENERGY STORAGE
4.1. MECHANICAL FLYWHEEL SYSTEMS
Flywheel systems are a versatile solution for short-term energy storage. The kinetic energy is given by Ek = ½ Jm 2. Here Jm is the moment of inertia of the rotating mass and the angular rotation frequency. More energy can be stored at higher frequency, but limits are given by the mechanical forces and safety considerations. In principle Jm can be made large without the necessity to use a large rotating mass. To maximize Jm the mass has to be concentrated at larger radii from the rotation axis. Modern designs of flywheels use light but strong materials like carbone fibers. Smaller flywheels are used today for energy recovery in cars and busses for public transportation.
For larger power loads and stored energy, as in the case of the CERN PS power supply, a flywheel system requires a significant investment. However, the system that was used at CERN (Fig. 3) has demonstrated a very reliable operation for many years with 6.8 million of cycles per year. In the CERN system the rotation speed varies by 5% over a cycle, and thus the stored energy by 10%. A generator is used to draw the energy from the mechanical system and convert it to electrical energy to power the accelerator magnet circuit. The same generator acts then as a motor to recover part of the energy stored in the magnetic fields of the magnets. Only the resistive energy losses in the magnet circuit have to be compensated by additional energy from the grid. In this way the system improves the overall energy efficiency.
A few years ago the flywheel system was replaced by a capacitive storage system. The new system has no moving parts, is thus potentially more reliable and exhibits smaller losses [4].
Fig. 3: Flywheel system in use at CERN over many years for operation of the PS synchrotron.
4.2. DIESEL GENERATOR
Many large research installations use diesel generators as backup systems to avoid brown outs in case of grid failure. In times of strongly varying supply of renewable energy sources on the grid, such devices may become attractive also for regular use to bridge periods of low supply, respectively periods of high cost of electrical energy. Diesel generators can cover the power range of a few kW for mobile units up to several MW for stationary units.
Grant Agreement 312453 PUBLIC 8 / 15 4.3. CAPACITORS
A capacitor stores energy in the electric field. The relation between voltage U and energy is given by E = ½ C U2. Thus the energy storage capability depends on the total electrical capacity of the system and the voltage that can be applied. The energy density per mass reaches roughly 300 J/kg.
Advantages are:
very high power, i.e. fast energy extraction possible
low losses result in high efficiency
no moving parts and no cryogenics required Disadvantages:
many capacitors are connected in parallel for most applications, thus the probability that one capacitor fails becomes significant
in case of a breakdown in one capacitor all energy would be deposited in this defect; to avoid this failure scenario safety fuses must be installed which complicates the system Capacitor banks are used for cycling synchrotrons and one example is the CERN POPS system that replaced the above mentioned flywheel. Another system was installed at J-PARC to provide the energy for cycling of the 30 GeV main ring synchrotron [5,6,7]. With a repetition rate of
1 Hz the magnet circuit of this synchrotron would cause load variations of 140 MW on the public grid, a magnitude that is not acceptable. The energy stored in the magnet circuit amounts to 10 MJ. After the beam cycle this energy is not lost but recovered in the capacitor bank. Only resistive losses in the magnets are converted to heat and are lost. As a result the energy consumption of the magnet circuit will be reduced to 30 % as compared to the case without the storage system.
The selection of the capacitors is a critical issue. Many capacitors contribute to the total capacity and thus the probability of failure must be very low for an individual capacitor. The lifetime of capacitors under operating conditions (temperature) should exceed 10 years. Typically a number of 108 cycles are required for application in a synchrotron. Ideally the failure of a single capacitor results not in a shortcut. J-PARC utilizes film capacitors which are composed of many small pixels to realize a “self-healing” concept. The failure of one pixel results in the electrical isolation of this pixel, a shortcut is avoided. The electrical capacity is only reduced by a small fraction, which is less than 1/10.000 of the original capacity. As result these capacitors do not fail abruptly, and their capacity is gradually decreased. Their lifetime can be defined using the degree of degradation, for example the time after which the capacity is reduced by 5 %.
Obviously the technology of these capacitors is still developed significantly and better technologies will likely appear in the future.
Grant Agreement 312453 PUBLIC 9 / 15 Fig. 4: Capacitor bank of the POPS system at CERN. Six containers with a total weight of 65 tons of capacitors are installed.
Modulators for klystrons store the energy for a pulse in capacitors. In this case the RF pulse length, i.e. the time during which the energy is withdrawn is relatively short and at maximum in the range of a few milliseconds. On the other hand the power provided to the RF system must be large. In the example of the European XFEL modulator, 17 MW are drawn from the capacitors in one unit, for a duration of 1.7 ms. Since the facility contains 27 units the total pulse power amounts to 450 MW, which equals a quarter of the average consumption of Hamburg. Clearly, it would not be possible to draw this power directly from the grid, the storage element is an essential part of the RF system.
4.4. SUPERCONDUCTING MAGNETIC STORAGE (SMES)
In this case the energy is stored in a magnetic field, generated by a superconducting coil. Energy and current are related through the inductance L of the coil: E = ½ L I 2. The superconducting coil ensures low ohmic losses and high field strength resulting in a high energy density for this energy storage method. Often a toroidal coil geometry is used to minimize stray fields. An overview on the SMES concept can be found in [8]. Although the superconductor itself has no electrical losses, power electronic is needed to insert or withdraw the energy of the SMES. The use as long-term storage therefore is not possible.
Advantages of SMES:
high efficiency for energy recovery
high power capability both for storing and recovering energy Disadvantages of SMES:
necessity of a cryogenic system and resulting investment cost
For accelerators the option of SMES has been investigated for modulators [10]. It was also considered for the cases of cycling synchrotrons at CERN and J-PARC, but was finally not adopted in favour of capacitor banks for economic reasons. Also the LIQHYSMES system,
Grant Agreement 312453 PUBLIC 10 / 15 described in the next section contains a SMES. While the SMES for modulators stores about one MJ of energy, for LIQHYSMES several GJ may be possible.
For comparison, the largest superconducting coil under discussion today is the coil of the ITER fusion reactor. With an inductance of 25H and a nominal current of 42 kA, the stored energy will amount to 22 GJ. Assuming a double inductance and the maximum possible current of 60 kA, even 90GJ = 25MWh of energy could be stored. Of course such a coil would be rather expansive, but as an example it sets the scale on what is feasible using magnetic storage.
Although the energy capacity seems large, it would be filled by 20 wind power stations, delivering 5MW each, within a quarter of an hour [9].
Thus the SMES concept is clearly suited for application in which fast, high power delivery or sinking is needed. A possible application for the use of the SMES would be for grid frequency primary or even secondary control. Here the power would be needed to regulate frequency changes of the grid [11]. But for larger amounts of energy other concepts involving chemical storage are needed.
Fig. 5: Toroidal SMES coil, developed at KIT.
4.5. LIQHYSMES
The LIQHYSMES [12, 13] is a hybrid energy storage consisting of a fast acting SMES and a slower hydrogen generation with a fuel cell/gas turbine generator; the Hydrogen will be liquefied to get a good energy to volume ratio. The SMES will be built with MgB2
superconductor. This material with a high Tc will have the advantage that the stored liquid Hydrogen (LH2) also acts as the coolant for the superconducting coils of the SMES. The general principle is shown in figure 6, a detailed concept of the storage unit is shown in figure 7.
Grant Agreement 312453 PUBLIC 11 / 15 Fig. 6: General concept of a LIGHYSMES. The fluctuations in the electrical grid are compensated by the SMES (fast) and conversion to and from Hydrogen (slow). Excess Hydrogen can be fed into a gas distribution grid (after conversion to CH4 if required)
Fig. 7: Storage unit of the LIQHYSMES. The magnet is cooled by the stored liquid Hydrogen. The heat exchanger is part of the liquefication process.
Grant Agreement 312453 PUBLIC 12 / 15 The hybrid energy storage has several advantages:
Large amounts of energy can be stored in the LH2 and is only limited by the size of the LH2 tank.
The fast acting properties of the SMES can quickly provide the required power to the load.
The SMES does not need to store more than 15 minutes worth of the rated power because the startup time of the H2 energy conversion (fuel cell, gas turbine or a combination of the two) will not take more time than that to reach the designated power level.
Excess energy can be extracted as liquid or gaseous Hydrogen for other applications (e.g. fuel cell cars, conversion to CH4 etc.)
Modular systems could be in different locations and be connected by a gaseous H2 pipeline.
As the main disadvantage one has to mention the high initial investment costs for the components i.e. liquefier, SMES and storage. For a 300 MW / 69 GWh LIGHYSMES with gas turbines one would have to calculate ~1900 €/kW ~8.25 €/kWh, smaller systems increase these numbers as the overhead does not scale linearly in addition standby losses also increase for smaller systems.
Despite the initial costs a LIGHYSMES provides a flexible and effective opportunity to store energy for accelerators.
5. RESEARCH AND DEVELOPMENT DIRECTIONS
In several fields, relevant for energy storage systems of accelerators, important developments are ongoing:
Capacitor Banks: The challenge is here to operate a parallel circuit of a huge number of capacitors. In case of a failure of one capacitor the entire system fails if no precautions are taken. Thus research is going into the direction of developing capacitors with lower fault rate and avoiding fault mechanisms that result in a shortcut of the capacitor.
SMES: The development of new superconductor cables can make SMES systems much more attractive. At higher temperature more efficient and cost effective cryogenic cooling solutions can be found.
Large capacity systems for energy management: The importance of large storage systems in context of energy management will increase with the fraction of renewable energy sources in the overall energy mix. The LIGHYSMES points the direction in which way large storage systems can be realised. Cost, capacity and efficiency of energy recovery are aspects that will be further optimized. As mentioned the integrated SMES can benefit much from the developments on superconducting cables.
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6. SUMMARY ON ENERGY STORAGE SYSTEMS FOR ACCELERATORS
Particle accelerators utilize energy storage systems in many areas. A range of different timescales, capacities and power rates is covered. Typically RF systems of linacs and magnet circuits of synchrotrons are cycled with high peak power and need storage systems. Storage technologies include mechanical systems and energy storage in electric or magnetic fields.
Chemical storage, for example using Hydrogen, can be used to realise large capacities. Besides the technical parameters also reliability, lifetime, possible failure scenarios are important.
Capacitive and inductive storage provide high efficiency and high power rates, but are limited in energy capacity and the cost is relatively high. The proposal of the LIQHYSMES system is an interesting option to combine the fast reaction time of inductive storage with the huge capacity of chemical storage. Moderately sized this system is a valid option for an uninterruptable power supply for an entire accelerator facility. Such supply would improve also the energy efficiency of facilities, since all facilities need considerable start-up times after a power failure, during which they are not productive but consume full power. As this example shows, storage systems may improve the overall energy efficiency of accelerator facilities in specific applications. However, also for standard applications like cycling synchrotrons, part of the energy is lost in the storage system and optimizations can be done.
Another aspect of increasing importance is energy storage in the context of energy management for large research facilities. With the introduction of more and more renewable energy sources the fluctuations of energy supply on the public grid will increase. Already today the cost of energy at the hourly spot market varies by an order of magnitude, indicating large fluctuations of energy production and consumption. Ideally one could use a large capacity storage system like the LIQHYSMES to bridge times of low availability. Today the high cost of large capacity storage systems make this option less attractive. However, in the future one may expect that this option comes closer to being economically valid, especially for new facilities which may include energy management in their initial plans. Work package 3 (EnEfficient) of the EUCARD-2 program includes a task on energy management that evaluates the possibilities to adjust the operation of facilities dynamically to the situation on the grid. In practise that means to shut down parts of the facility during times of high demand, but to ensure effective and quick start-up times. This scenario of a “virtual power plant” could be combined with energy storage to allow for more operational flexibility. In any case more research should be invested in such large capacity storage systems for energy management, to make them more cost effective.
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7. REFERENCES
[1] EuCARD-2: Enhanced European Coordination for Accelerator Research & Development, 2013-17, http://eucard2.web.cern.ch/
[2] Energy Efficiency of Particle Accelerators (EnEfficient),“ 2013-17, http://www.psi.ch/enefficient/
[3] Johanna Torberntsson, ESS, Cooling related inventory of accelerator facilities in Europe, https://edms.cern.ch/ui/file/1325126/4/EuCARD2-Del-D3-1-Final.pdf
[4] J.-P. Burnet, CERN, A novel 60 MW Pulsed Power System based on Capacitive Energy Storage, Workshop Energy for Sustainable Science, Lund (2011)
[5] H. Sato et al, Application of Energy Storage System for the Accelerator Magnet Power Supply, Kyoto, IPAC (2010),
http://accelconf.web.cern.ch/AccelConf/IPAC10/papers/wepd061.pdf
[6] Y. Morita, KEK/J-PARC, MR Upgrade and Power Management, Workshop on the efficiency of proton driver accelerators, PSI (2016),
http://indico.psi.ch/getFile.py/access?contribId=23&sessionId=4&resId=3&materialId=slides
&confId=3848
[7] Y. Kurimoto, KEK/J-PARC, Development of new high slew-put and high energy- efficient power supplies for the J-Parc, DESY (2015)
https://indico.desy.de/getFile.py/access?contribId=61&sessionId=15&resId=0&materialId=sli des&confId=11870
[8] P. Tixador, Grenoble INP - Institut Néel / G2Elab, SMES – Present Status and Future, Workshop Energy for Sustainable Science, Lund (2011)
[9] H.-J. Eckholdt, M.Terörde, DESY, SMES usage and power grid applications at RIs, CERN (2013)
https://indico.cern.ch/event/245432/contributions/1566070/attachments/420450/583844/CER N_Workshop_2013.pdf
[10] D. A. Edwards (Editor), TESLA Test Facility Linac Design Report, 1995, http://tesla.desy.de/TTF_Report/CDR/pdf/cdr_contents.pdf
[11] M. Terörde, Einsatz eines supraleitenden magnetischen Energiespeichers zur Primärregelung bei DESY. Masterarbeit Fernuniversität Hagen, 2012
[12] LIQHYSMES - A Novel Energy Storage Concept for Variable Renewable Energy Sources Using Hydrogen and SMES, IEEE Transactions on Applied Superconductivity, Vol. 21 No. 3, June 2011
[13] M. Sander, R. Gehring, H. Neumann, LIQHYSMES – A 48 GJ Toroidal MgB2-SMES for Buffering Minute and Second fluctuations, IEEE Transactions on applied Superconductivity, Vol. 23 No. 3, June 2013
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ANNEX: GLOSSARY
Acronym Definition
SMES Superconducting Magnetic Energy Storage
LIQHYSMES LIQuid HYdrogen (LH2) and Superconducting Magnetic Energy Storage
TESLA Tera electron volt Energy Superconducting Linear Accelerator
