A New Generation of Pulsed Power Supplies for Experimental Physics Based on Supercapacitors Giuseppe Maffia, Alessandro Lampasi, Pietro Zito National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) Frascati (RM), Italy
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[email protected] Abstract—Big physics experiments often require high power peaks for a short duration with low duty cycle. Even though the effective energy may be relatively small, the power supplies of these experiments can be critical from the technical, economic, and environmental points of view. This paper shows that the supercapacitor technology can simplify many power supply systems and can extend the scenarios explored by the experiments. Supercapacitors were already proposed and introduced for many applications, but their use is particularly advantageous in big physics experiments, especially when the current is high and the voltage is relatively low. This statement will be supported by some models and case studies based on real experimental devices (operating or that are going to be commissioned) with currents up to 60 kA. The presented designs based on supercapacitors simplify the existing apparatus, reduce the costs, increase the local power availability, and even extend the possible experimental activities. The reported analyses and results are mainly focused on nuclear fusion researches, but can be easily applied to many other fields. Keywords—AC/DC converter; physics experiment; high current; plasma physics; pulsed power supply; supercapacitor
I. INTRODUCTION Supercapacitors (SCs), also referred as ultracapacitors, are a family of devices featuring exceptional capacitance values, up to several orders of magnitude higher than conventional capacitors [1-7]. Such characteristic opened new scenarios, increasing the performances achievable by electrical and electronic systems and extending the possible fields of application of capacitors. It is well-known that electrical energy can be stored only by a transformation to another form of energy, as electrochemical (batteries, fuel cells, flow batteries), electromagnetic (capacitors, coils), kinetic (flywheels), thermodynamic (compressed air) or potential (pumped water), and so on. Until recently, batteries were the only simple option for energy storage. Nowadays, SCs can offers an alternative with higher power density and faster charge time, while their longer lifetime can compensate their higher costs with respect to batteries. On the other hand, SCs have a significantly lower energy density than the batteries. For this reason, instead of replacing batteries they can be used in conjunction. In particular, SCs can sustain the load peaks for systems continuously fed by batteries, reducing the system size and consumption.
Since several technical and economic problems could solved by exploiting the SC technology, many applications of SCs were proposed in the literature, ranging from smart grid to electrical vehicles. However, the advantages of SCs are particularly evident in physics experiment with high pulsed power, up to tens or hundreds of megawatts [8-12]. These values can be obtained only under special power delivery contracts or energy storage devices. This aspect is particularly critical in nuclear fusion and plasma physics applications. In fact, this kind of facilities can be realized only in locations with adequate power (even the maximum of the entire country). The realization of a new facility is subject to the preventive analysis of the power available in the selected location. In general, the fusion-oriented facilities are connected either to the national grid at the maximum available voltage or are supported by large flywheel machines for energy storage [13, 14]. In these cases, even though the power is high, the energy may be relatively small due to the short operation time with a long repetition time [8-11]. Moreover, the power profile and duty-cycle are known in advance, at least with a good approximation. For example, in fusion experiments the operators select a desired “scenario” containing in practice the time history of all the currents and voltages generated by the power converters. Therefore, the use of SC-based systems may simplify the power supplies, reduce the costs, increase the local power availability and even extend the performances and the possible experiments [12, 13]. This paper proves these statements by the case studies summarized in Table I and based on the authors’ experience. These are not theoretical examples but refer to ongoing experiments. In particular, the following solutions were developed: 1.
An alternative approach for the power supplies of the MULTI-PINCH experiment [9].
2.
A possible realization of the very demanding power supply for the PROTO-SPHERA experiment [9].
3.
An alternative scheme to obtain the fast current variations necessary for the plasma breakdown [10].
4.
An analysis to integrate a SC-based energy storage in an entire big-physics facility, as a tokamak [13].
The developed analyses are not only technical, but include also the necessary economical assessments.
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TABLE I. SUMMARY OF THE MAIN CONSIDERED CASE STUDIES WITH APPROXIMATE CHARACTERISTICS. SINCE FTU AND DTT INCLUDE MANY POWER SYSTEMS, ONLY THE MOST RELEVANT VALUES ARE REPORTED. Case Study
Current
Voltage
Duty cycle
MULTI-PINCH coil
10 kA
350 V
1/600 s/s
MULTI-PINCH arc
10 kA
350 V
1/600 s/s
PROTO-SHPERA arc
60 kA
350 V
1/600 s/s
Switching network unit
20 kA
5 kV
≈10/1800 s/s
CARM radiofrequency source
30 A
700 kV
≈100/1 µs/s
Frascati Tokamak Upgrade (FTU)
±25 kA
±5 kV
≈3/900 s/s
Divertor Test Tokamak (DTT)
±40 kA
±300 V
100/3600 s/s
II. PRACTICAL CHARACTERISTICS OF SCS A. Benefits The main characteristics of SCs are summarized in the following: •
High specific capacitance (more than 10 F/g).
•
High specific power (more than 5 W/g).
•
Low equivalent series resistance (ESR), contributing to fast power release.
•
Fast charge time (from some seconds to some minutes) and slow auto-discharge time (some days).
•
Characteristics not significantly affected by the state of charge.
•
Efficiency higher than 90%.
•
Lifetime up to 1 million of charge/discharge cycles and up to 15 years, namely orders of magnitude better than batteries due to absence of chemical reactions.
•
Stable performances over large temperature range (-40 to 80 °C).
•
No heavy metals or special disposal.
•
Even though asymmetric SCs exist, standard SCs have no a true polarity. Unlike other capacitor families, reverse-charging a SC may lower the capacity, but without catastrophic consequences.
B. Drawbacks and Constrains SCs have inherent limitations, that were partially overcame by the technology progresses. The most relevant constrains concern the maximum voltage. Considering the formula for the capacitance
The limited voltage also affects the maximum energy that can be stored in a SC, that is .
(2)
The use of SCs for higher voltages is possible only by series/parallel configurations. However, due to the capacitance and ESR unbalancing, it is necessary to introduce also further active or passive components to stabilize the voltage, thus increasing the global costs and complexity. In order to mitigate these inconveniences, manufactures provide assembled modules reaching hundreds of volts and hundreds of farads. SCs cannot be modelled by a single capacitance, even including the ESR. A more complex model is necessary to take into account all the electrochemical processes. The definition of such a model is still subject of research with several proposals [15-17], also because SCs are rather recent components with ongoing developments. It is useful to notice that a model based on lumped circuit components (resistances, capacitances, switch) is used by some manufacturers and was recently introduced by the PSIM simulation software. C. Cost Considerations The present costs of SCs are in the order of 10 €/Wh (≈2.5 m€/J) and 0.01 €/F and are expected to drop in the future. For a quick comparison, the cost of battery-based accumulators is around 0.15 €/Wh (≈0.04 m€/J), even with a higher specific energy. However, the longer lifetime of SCs can compensate this spread: simple calculations can show that the breakeven point for the two technologies can be reached in 4 years. Moreover, batteries have significantly worst performances and shorter lifetime (number of charging cycles) in case of improper charge variations. On the other hand, if the introduction of proper energy storage systems is used as an alternative to a connection to the distribution grid, the cost benefits are evident. III. A PRACTICAL USE OF THE RAGONE CHART The functional characteristics of the energy storage technologies can be characterized by: 1.
The amount of storable energy.
2.
The rate at which such energy can be acquired or released (available power transfer).
A Ragone chart as in Fig. 1 provides a quick tool to summarize these items, normalized with respect to the device mass [1, 18-19]. This is particularly useful for the case of electrical vehicles and portable devices.
(1)
The ellipses in Fig. 1 summarizes the most relevant storage technologies. The zones are only indicative of a family of devices. Further curves could be inserted for a more specific type of device (for instance, type of battery or capacitor, as reported in Fig. 1 for some cases).
the high SC capacitance is obtained by an extremely large equivalent surface S and by a minimal (double-layer) distance d
